Environmental Encyclopedia

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Environmental Encyclopedia 3

Fluidized bed combustion

Westrup, Hugh. “Debugging a Killer Virus.” Current Science 84, no. 9

(January 8, 1999): 4.

Flue gas

The exhaust gas vented from

combustion

, a chemical reac-

tion, or other physical process, which passes through a duct

into the

atmosphere

. Exhaust air is usually captured by an

enclosure and brought into the exhaust duct through induced

or forced ventilation. Induced ventilation is created by low-

ering the pressure in the duct using fans at the end of the

duct. Forced ventilation occurs when exhaust air is forced

into the duct using high pressure inlet air. Flues are valuable

because they not only direct polluted air to a

pollution

control

device, but also keep the air pollutant concentrations

high. High concentrations can be important if the air pollut-

ant removal process is concentration dependent. See also Air

pollution control

Flue-gas scrubbing

Flue-gas scrubbing is a process for removing oxides of sulfur

and

nitrogen

from the waste gases emitted by various indus-

trial processes. Since the oxides of sulfur and nitrogen have

been implicated in a number of health and environmental

problems, controlling them is an important issue. The basic

principle of scrubbing is that flue gases are forced through

a system of baffles within a smokestack. The baffles contain

some chemical or

chemicals

that remove pollutants from

these gases.

A number of scrubbing processes are available, all of

which depend on the reaction between the oxide and some

other chemical to produce a harmless compound that can

then be removed from the smokestack. For example, cur-

rently the most common scrubbing reaction involves the

reaction between

sulfur dioxide

and lime. In the first step

of this process, limestone is heated to produce lime. The

lime then reacts with sulfur dioxide in flue gases to form

calcium sulfite, which can be removed with

electrostatic

precipitation

Many other scrubbing reactions have been investi-

gated. For example, magnesium oxide can be used in place

of calcium oxide in the scrubber. The advantage of this

reaction is that the magnesium sulfite that is formed decom-

poses readily when heated. The magnesium oxide that is

regenerated can then be reused in the scrubber while the

sulfur dioxide can be used to make sulfuric

acid

.Inyet

another process, a mixture of sodium citrate and citric acid

is used in the scrubber. When sulfur dioxide is absorbed by

the mixture, a reaction occurs in which elemental sulfur is

precipitated out.

567

Although the limestone/lime process is by far the most

popular scrubbing reaction, it has one serious disadvantage.

The end product, calcium sulfite, is a solid that must be

disposed of in some way.

Solid waste

disposal is already a

serious problem in many areas, so adding to that problem

is not desirable. For that reason, reactions such as those

involving magnesium oxide, sodium citrate and citric acid

have been carefully studied. The products of these reactions,

sulfuric acid and elemental sulfur, are valuable raw materials

that can be sold and used. In spite of that fact, the limestone/

lime scrubbing process, or some variation of it, remains the

most popular method of extracting sulfur dioxide from flue

gases today.

Scrubbing to remove

nitrogen oxides

is much less

effective. In principle, reactions like those used with sulfur

dioxide are possible. For example, experiments have been

conducted in which ammonia or

ozone

is used in the

scrubber to react with and remove oxides of nitrogen. But

such methods have had relatively little success and are rarely

used by industry.

Flue gas

scrubbing has long met with

resistance

from

utilities and industries. For one thing, they are not convinced

that oxides of sulfur and nitrogen are as dangerous as envi-

ronmentalists sometimes claim. In addition, they argue that

the cost of installing

scrubbers

is often too great to justify

their use. See also Air pollution control; Dry alkali injection;

Stack emissions

[David E. Newton]

ESOURCES

OOKS

American Chemical Society. Cleaning Our Environment: A Chemical Perspec-

tive. 2nd edition. Washington, DC: American Chemical Society, 1978.

ERIODICALS

Bretz, E. A. “Efficient Scrubbing Begins With Proper Lime Prep, Han-

dling.” Electrical World 205 (March 1991): 21–22.

“New Choices in FGD Systems Offer More Than Technology.” Electrical

World 204 (November 1990): 46–47.

“Scrubbers, Low-Sulfur Coal, of Plant Retirements?” Electrical World 204

(June 1990): 18.

Fluidized bed combustion

Of all

fossil fuels

coal

exists in the largest amount. In fact,

the world’s coal resources appear to be sufficient to meet

our energy needs for many hundreds of years. One aim of

energy technology, therefore, is to find more efficient ways

to make use of these coal reserves. One efficiency procedure

that has been under investigation for at least two decades is

known as fluidized bed

combustion

Environmental Encyclopedia 3

Fluoridation

Fluidized bed combustion. Fuel is lifted by a

stream of air from underneath the bed. Fuel

efficiency is good and sulfur dioxide and nitro-

gen oxide emissions are lower than with con-

ventional boilers. (McGraw-Hill Inc. Reproduced by

permission.)

In a fluidized bed boiler, granulated coal and limestone

are fed simultaneously onto a moving grate. A stream of air

from below the grate lifts coal particles so that they are

actually suspended in air when they begin to burn. The flow

of air, small size of the coal particles, and exposure of the

particles on all sides contribute to an increased rate of com-

bustion. Heat produced by burning the coal is then used to

boil water, run a turbine, and drive a generator, as in a

conventional power plant.

The fluidized bed process has much to recommend

it from an environmental standpoint. Sulfur and

nitrogen

oxides

react with limestone added to the boiler along with

the coal. The product of this reaction, primarily calcium

sulfite and calcium sulfate, can be removed from the bottom

of the boiler. However, disposing of large quantities of this

waste product represents one of the drawbacks of the fluid-

ized bed system.

Fly ash

is also reduced in the fluidized bed process.

As coal burns in the boiler, particles of fly ash tend to adhere

to each other, forming larger particles that eventually settle

out at the bottom of the boiler. Conventional methods of

568

removal in the stack, such as

electrostatic precipitation

, can

further increase efficiency with which particles are removed.

Incomplete combustion of coal is common in the fluid-

ized bed process. However,

carbon monoxide

and

hydro-

gen

sulfide formed in this way are further oxidized in the

space above the moving grate. The products of this further

oxidation are then removed by the limestone (which reacts

with

sulfur dioxide

) or allowed to escape harmlessly in to

the air (in the case of the

carbon dioxide

). After being

used as a

scavenger

in the process, the limestone, calcium

sulfite, and calcium sulfate can be treated to release sulfur

dioxide and regenerate the original limestone. The limestone

can then be re-used and the sulfur dioxide employed to make

sulfuric

acid

A further advantage of the fluidized bed process is

that it operates at a lower temperature than does a conven-

tional power plant. Thus, the temperature of cooling water

ejected from the plant is lower, and the amount of

thermal

pollution

of nearby waterways correspondingly lessened.

Writers in the 1970s expressed high hopes for the

future of fluidized bed combustion systems, but the cost of

such systems is still at least double that of a conventional

plant. They are also only marginally more efficient than

a conventional plant. Still, their environmental assets are

obvious. They should reduce the amount of sulfur dioxide

emitted by up to 90% and the amount of

nitrogen

oxides

by more than 60%. See also Air pollution control; Stack

emissions

[David E. Newton]

ESOURCES

OOKS

Electric Power Research Institute. Atmospheric Fluidized Bed Combustion

Development. Palo Alto, CA: EPRI, 1982.

Government Institutes, Inc. Staff, eds. Evaluating the Fluidized Bed Com-

bustion Option, 1988. Rockville, MD: Government Institutes, 1988.

Marshall, A. R. Reduced NOx Emissions and Other Phenomena in Fluidized

Bed Combustion. Lanham: UNIPUB, 1992.

ERIODICALS

Balzhiser, R. E., and K. E. Yeager. “Fluidized Bed Combustion.” Scientific

American 257 (September 1987): 100–107.

Fluoridation

Fluoridation is the precise adjustment of the concentration

of the essential trace element fluoride in the public water

supply to protect teeth and bones. Advocates of fluoridation

such as the American Dental Association (ADA) and the

National Center for Chronic Disease Prevention and Health

Promotion (CDC) state that fluoridation is a safe and effec-

tive method of preventing tooth decay. Opponents of fluori-

Environmental Encyclopedia 3

Fluoridation

dation however, such as Citizens for Health and the Fluoride

Action Metwork, maintain that the role fluoridation in the

decline of tooth decay is in serious doubt and that more

research is required before placing a compound in water

reservoirs that could cause

cancer

, brittle bones, and neuro-

logical problems.

Fluoride is any compound that contains fluorine, a

corrosive, greenish-yellow element. Tooth enamel contains

small amounts of fluoride. In addition, fluoride is found in

varying amounts in water and in all food and beverages,

according to the ADA.

A Colorado dentist discovered the effects of fluoride

on teeth in the 1900s. When Frederick McKay began prac-

ticing in Colorado Springs, he established a connection be-

tween a substance in the water and the condition of residents’

teeth. People did not have cavities, but their teeth were

stained brown. Dental research started on the substance that

was identified as fluoride during the 1930s. Researchers

concluded that a concentration of fluoride in drinking water

at a ratio of 1 part per million (ppm) prevented tooth decay

without staining teeth. In 2000, the ADA stated that a

fluoride concentration ranging from 0.7 ppm to 1.2 ppm

was sufficient to fight tooth decay.

The first community to try fluoridation was Grand

Rapids, Michigan. The city fluoridated the community water

supply in 1945. Ten years later, Grand Rapids reported that

incidents of tooth decay had declined by 60% in the children

raised on fluoridated water. During the 1950s, Chicago,

Philadelphia, and San Francisco also started to fluoridate

their water supply. Cities including New York and Detroit

opted for fluoridation during the 1960s. However, not all

Americans advocated fluoridation. During the 1950s and

1960s, members of the John Birch Society maintained that

fluoridation was a form of mass medication by the govern-

ment. Some members charged that fluoridation was part of

a Communist plot to take over the country. In the decades

that followed, fluoridation was no longer associated with

conspiracy theories. However, opinion about fluoridation

was divided at the close of the twentieth century. By 2000,

public water systems served 246.1 million Americans, ac-

cording to the federal CDC. Of that amount, 65.8% of

Americans used fluoridated water.

In Washington D.C., 100% of the water is fluoridated,

according to a CDC report on the percentage of state popula-

tions with fluoridated public water systems in 2000. The top

10 on the list were: Minnesota (98.2%), Kentucky (96.1%),

North Dakota (95.4%), Indiana (95.3%), Tennessee (94.5%),

Illinois (93.4%), Virginia (93.4%), Georgia (92.9%), Iowa

(91.3%), and South Carolina (91.2%). At the other end of

the spectrum in terms of fluoridated public water usage were:

Louisiana (53.2%), Mississippi (46%), Idaho (45.4%), New

Hampshire (43%), Wyoming (30.3%), California (28.7%),

569

Oregon (22.7%), Montana (22.2%), New Jersey (15.5%),

Hawaii (9%), and Utah (2%). The CDC estimated the cost

of fluoridation at 50 cents per year in communities of more

than 20,000 residents. The annual cost was estimated at $1 in

communities of 10,000 to 20,000 residents. In communities

numbering less than 5,000 people, the yearly cost was esti-

mated at $3.

The CDC reported in 2000 that extensive research

during the previous 50 years proved that fluoridation was

safe. Fluoridation was also endorsed by groups including the

American Medical Association, the American Academy of

Pediatrics, the National PTA, and the American Cancer

Society. Advocates of fluoridation state that it especially

benefits people who may not be able to afford dental care.

Opponents however, counter that toothpaste with fluoride

is available for people who believe that fluoride fights tooth

decay. Furthermore, opponents point out that toothpaste

with fluoride contains a warning label advising users to “seek

professional assistance or contact a poison control center” if

they accidentally swallow more than the amount used for

brushing teeth. Lastly, their question the research methodol-

ogy used to conclude that fluoridation is responsible for

decreased tooth decay. Fluoridation critics include consumer

advocates

Ralph Nader

and Jim Turner. Turner chairs the

board of Citizens for Health, a grassroots organization that

is asking Congress to hold hearings and review fluoridation

policy. Citizens for Health belongs to the groups that believe

more research is required to determine the risks and benefits

of fluoridation.

[Liz Swain]

ESOURCES

OOKS

American Water Works Association. Water Fluoridation Principles and

Practices. Denver: AWWA, 1996.

Health Research Staff. Facts You Should Know About Fluoridation. Pomeroy,

WA: Health Research Books, 1996.

Martin, Brian. Scientific Knowledge in Controversy: The Social Dynamics of

the Fluoridation Debate (Science, Technology and Society). Albany, NY: SUNY

Press, 1991.

RGANIZATIONS

American Dental Association, 211 E. Chicago Avenue, Chicago, IL USA

60611 (312) 440-2500, Fax: (312) 440-2800, Email: publicinfo@ada.org,

<http://www.ada.org>

Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta,

GA USA 30333 (404) 639-3534, Toll Free: (800) 311-3435, , <http://

www.cdc.gov>

Citizens for Health, 5 Thomas Circle, NW, Suite 500, Washington, D.C.

USA 20005 (202) 483-1652, Fax: (202) 483-7369, Email: cfh@citizens.org,

<http://www.citizens.org>

National Center for Fluroridation Policy and Research (NCFPR) at the

University of Buffalo, 315 Squire Hall, Buffalo, NY USA 14214 (716)

829-2056 , Fax: (716) 833-3517, Email: mweasley@buffalo.edu, <http://

fluoride.oralhealth.org>

Environmental Encyclopedia 3

Fly ash

The fine ash from

combustion

processes that becomes dis-

persed in the air. To the casual observer it might appear as

smoke

, and indeed it is often found mixed with smoke. Fly

ash arises because fuels contain a small fraction of incombus-

tible matter. In a fuel like

coal

, ash has a rock-like siliceous

composition, but the high temperatures at which it is formed

often means that metals such as iron are incorporated into

the ash particles, which take on the appearance of small,

colored, glassy spheres.

Petroleum

produces less ash, but it

is often associated with a range of oxides such as vanadium

(in the case of fuel oils) and, more noticeably, hollow spheres

carbon

. In traditional furnaces, much ash remained on

the grate, but modern furnaces produce such fine ash that

it is carried away in the hot exhaust gas.

Early power stations dispersed so much fine fly ash

throughout the nearby

environment

that they were soon

forced to adopt early

pollution

abatement techniques. They

adopted “cyclones” in which centrifugal force removes the

particles by causing the waste gas stream to flow on a curved

or vortical course. This technique is effective down to particle

sizes of about 10 ␮m, but smaller particles are not removed

well by

cyclone

collectors and here

electrostatic precipita-

tion

process often proves more successful, coping with a size

range 30—0.1 ␮m. In electrostatic precipitators the particles

are given a negative charge and then attracted to a positive

electrode where they are collected and removed. Cloth or

paper

filters

and spraying water through the exhaust gases

can be useful in removing fly ash.

Fly ash is a nuisance at high concentrations because

it accumulates as grit on the surfaces of buildings, clothes,

cars, and outdoor furnishings. It is a highly visible and very

annoying aspect of industrial

air pollution

. The deposition

of fly ash increases cleaning costs incurred by people who

live near poorly controlled combustion sources. Fly ash also

has health impacts because the finer particles can penetrate

into the human lung. If the deposits are especially heavy,

fly ash can also inhibit plant growth.

Each year millions of tons of fly ash are produced

from coal-powered furnaces, most of which are dumped in

waste tips. Care needs to be taken that toxic metals and

alkalis are not leached from these disposal sites into water-

courses. Fly ash may be used as a low grade cement in road

building because it contains a large amount of calcium oxide

but generally, the demand is rather low.

[Peter Brimblecombe Ph.D.]

ESOURCES

OOKS

Sellers, B. H. Pollution of Our Atmosphere. Bristol: Adam Hilger, 1984.

570

Flyway

The route taken by migratory birds and waterfowl when

they travel between their breeding grounds and their winter

homes. Flyways often follow geographic features such as

mountain ranges, rivers, or other bodies of water. Protecting

flyways is one of the many responsibilities of

wildlife

manag-

ers. Draining of

wetlands

, residential and commercial de-

velopment, and

overhunting

are some of the factors that

threaten flyway sites visited by birds for food and rest during

migration

. In most cases, international agreements are

needed to guarantee protection along the entire length of a

flyway. In the United States, flyway protection is financed

to a large extent by funds produced through the Migratory

Bird

Hunting

Stamp Act passed by the U.S. Congress in

1934.

Food additives

Food additives are substances added to food as flavorants,

nutrients, preservatives, emulsifiers, or colorants. In addition,

foods may contain residues of

chemicals

used during the

production of plant or animal crops, including pesticides,

antibiotics, and growth hormones. The use of most food

additives is clearly beneficial because it results in improved

public health and prevention of spoilage, which enhances

the food supply. Nevertheless, there is controversy about the

use of many common additives and over the presence of

contaminants in food. This is partly because some people

are hypersensitive and suffer allergic reactions if they are

exposed to certain of these chemicals. In addition, some

people believe that low levels of chronic toxicity and diseases

may be caused in the larger population by exposure to some of

these substances. Although there is no compelling scientific

evidence that this is indeed the case, the possibility of chronic

damage caused by food additives and chemical residues is

an important social and scientific issue.

The use of food additives in the United States is closely

regulated by the government agencies responsible for health,

consumer safety, and agriculture. This is also the case of

other developed countries in Europe, Canada, and elsewhere.

Chemicals cannot be used as additives in those countries

unless regulators are convinced that they have been demon-

strated to be toxicologically safe, with a wide margin of

security. In addition, chemicals added to commercially pre-

pared foods must be listed on the packaging so that consum-

ers can know what is present in the foodstuffs that they

choose to eat. Because of the intrinsic nature of low-level,

toxicological risks, especially those associated with diseases

that may take a long time to develop, scientists are never

able to demonstrate that trace exposures to any chemical are

absolutely safe—there is always a level of risk, however small.

Environmental Encyclopedia 3

Food additives

UNHEALTHY FOOD ADDITIVES

Name Description Example products

Aspartame An artificial sweetener associated with Diet sodas, sugar substitutes, etc.

rashes, headaches, dizziness,

depression, etc.

Brominated vegetable oil (BVO) Used as an emulsifier and clouding Sodas, etc.

agent. Its main ingredient, bromate, is

a poison.

Butylated hydroxyanisole (BHA)/ Prevents rancidity in foods and is Cereal and cheese packaging

butylated hydroxytoluene (BHT) added to food packagings. It slows the

transfer of nerve impulses and affects

sleep, aggressiveness, and weight in

test animals.

Citrus red dye #2 Used to color oranges, it is a probable Oranges

carcinogen. The FDA has

recommended it be banned.

Monosodium gltamate (MSG) A flavor enhancer that can cause Fast food, processed and

headaches, heart palpitations, and packaged food

nausea.

Nitrites Used as preservatives, nitrites form Cured meats and wine

cancer-causing compounds in the

gastrointestinal tract and have been

associated with cancer and birth

defects.

Saccharin An artificial sweetener that may be Diet sodas and sugar substitutes

carcinogenic.

Sulfites Used as a food preservative, sulfites Dried fruits, shrimp, and frozen

have been linked to at least four deaths potatoes

reported to the FDA in the United

States.

Tertiary butyhydroquinone It is extremely toxic in low doses and Candy bars, baking sprays, and

(TBHQ) has been linked to childhood fast foods

behavioral problems.

Yellow dye #6 Increases the number of kidney and Candy and sodas

adrenal gland tumors in lab rats. It has

been banned in Norway and Sweden.

571

Environmental Encyclopedia 3

Food additives

Because some people object to these potential, low-level,

often involuntary risks, a certain degree of controversy will

always be associated with the use of food additives. This is

also true of the closely related topic of residues of pesticides,

antibiotics, and growth hormones in foods.

Flavorants

Certain chemicals are added to foods to enhance their

flavor. This is particularly true of commercially processed or

prepared foods, such as canned vegetables and frozen foods

and meals. One of the most commonly added flavorants is

table salt (or sodium chloride), a critical

nutrient

for humans

and other animals. In large amounts, however, sodium chlo-

ride can predispose people to developing high blood pressure,

a factor that is important in strokes and other circulatory

and heart diseases.

Table sugar (or sucrose), manufactured from sugar

cane or sugar beets, and fructose, or fruit sugar, are com-

monly used to sweeten prepared foods. Such foods include

sugar candies, chocolate products, artificial drinks, sweet-

ened fruit juices, peanut butter, jams, ketchup, and most

commercial breads. Sugars are easily assimilated from foods

and are a useful form of metabolic energy. In large amounts,

however, sugars can lead to weight gain, tooth decay, and

hypoglycemia and diabetes in genetically predisposed people.

Artificial sweeteners such as saccharine, aspartame, and so-

dium cyclamate avoid the nutritional problems associated

with eating too much sugar. These nonsugar sweeteners may

have their own problems, however, and some people consider

them to be a low-level health hazard.

Monosodium glutamate (or MSG) is commonly used

as a flavor enhancer, particularly in processed meats, prepared

soups, and oriental foods. Some people are relatively sensitive

to this chemical, developing headaches and other symptoms

that are sometimes referred to as “Chinese food syndrome.”

Other flavorants used in processed foods include many kinds

of spices, herbs, vanilla, mustard, nuts, peanuts, and wine.

Some people are extremely allergic to even minute exposures

to peanuts or nuts in food and can rapidly develop a condition

known as anaphylactic shock, which is life-threatening un-

less quickly treated with medicine. This is one of the reasons

why any foods containing peanuts or nuts as a flavoring

ingredient must be clearly labeled as such.

Many flavorants are natural in origin. Increasingly,

however, synthetic flavorants are being discovered and used.

For example, vanilla used to be extracted from a particular

species

of tropical orchid and was therefore a rather expen-

sive flavorant. However, a synthetic vanilla flavorant can

now be manufactured from wood-pulp lignins, and this has

made this pleasant flavor much more readily available than

it used to be.

572

Nutrients

Many foods are fortified with minerals, vitamins, and

other micronutrients. One such example is table salt, which

has iodine added (as potassium iodide) to help prevent

goiter

in the general population. Goiter used to be relatively com-

mon but is now rare, in part because of the widespread use

of iodized salt.

Other foods that are commonly fortified with minerals

and vitamins include milk and margarine (with vitamins A

and D), flour (with thiamine, riboflavin, niacin, iron), and

some commercial breads and breakfast cereals (with various

vitamins and minerals, particularly in some commercial ce-

real preparations). Micronutrient additives in these and other

commercial foods are carefully formulated to help contribute

to a balanced diet in their consumers. Nevertheless, some

people believe that it is somehow un-natural and un-healthy

to consume foods that have been adulterated in this manner,

and they prefer to eat “natural” foods that do not have any

vitamins or minerals added to them.

Preservatives

Preservatives are substances added to foods to prevent

spoilage caused by bacteria,

fungi

, yeasts, insects, or other

biological agents. Spoilage can lead to a decrease in the

nutritional quality of foods, to the growth of food-poisoning

microorganisms

such as the botulism bacterium, or to the

production of deadly chemicals, such as

aflatoxin

, that can

be produced in stored grains and seeds (such as peanuts) by

species of fungi.

Salt has long been used to preserve meat and fish,

either added directly to the surface or by immersing the food

in a briny solution.

Nitrates and nitrites

(such as sodium

nitrate, or saltpetre) are also used to preserve meats, especially

cured foods such as sausages, salamis, and hams. These

chemicals are especially useful in inhibiting the growth of

Clostridium botulinum, the bacterium that causes deadly bot-

ulism. Vinegar and wood

smoke

are used for similar pur-

poses.

Sulfur dioxide

, sodium sulfite, and benzoic

acid

are

often used as preservatives in fruit products, such beverages

as wine and beer, and in ketchup, pickles, and spice prepara-

tions.

Anti-oxidants are chemicals added to certain foods to

prevent a deterioration in their quality or flavor, occurring

due to the exposure of fats and oils to atmospheric oxygen.

Examples of commonly used antioxidants are ascorbic acid

(or vitamin C), butylated hydroxyanisole (or BHA), butyl-

ated hydroxytoluene (or BHT), gallates, and ethoxyquin.

Stabilizers and emulsifiers

Stabilizers and emulsifiers are added to prepared foods

to maintain suspensions of fats or oils in water matrices (or

vice versa), or to prevent the caking of ingredients during

storage or preparation. One example of an emulsifying addi-

tive is glyceryl monostearate, often added to stored starch

Environmental Encyclopedia 3

Food and Drug Administration

products to maintain their texture. Alginates are compounds

added to commercial ice cream, salad dressing, and other

foods to stabilize emulsions of oil- or fat-in-water during

storage.

Colorants

Some prepared foods have colors added to “improve”

their aesthetic qualities and thereby to make them more

attractive to consumers. This practice is especially common

in the preparation of confectionaries such as candies, choco-

late bars, ice creams, and similar products, and in fancy

cakes and pastries. Similarly, products such as ketchup and

strawberry preserves have red dyes added to enhance their

color, relishes and tinned peas have green colors added, and

dark breads may contain brown colorants. Most margarines

have yellow colors added, to make them appear more similar

to butter. Artificial drinks and drink-mixes contain food

colorants appropriate to their flavor--cherry and raspberry

contain red dyes, and so forth.

Various chemicals are used as food colorants, some of

them being extracted from plants (for example, yellow and

orange carotenes), while many others are synthetic chemicals

derived from

coal

tars and other organic substances. The

acute toxicity (i.e., short-term

poisoning

) and chronic toxic-

ity (i.e., longer-term damage associated with diseases, can-

cers, and developmental abnormalities) of these colorants

are stringently tested on animals in the laboratory, and the

substances must be demonstrated to be safe before they are

allowed to be used as food additives. Still, some people

object to having these chemicals in their food, and choose

to consume products that are not adulterated with colorants.

Residues of pesticides, antibiotics, and growth hor-

mones

Insecticides, fungicides, herbicides, and other pesti-

cides are routinely used in modern, industrial agriculture.

Some of these chemicals are persistent, because they do

not quickly break down in the

environment

to simpler

substances, and/or they do not readily wash off produce.

The chemicals in such cases are called residues, and it is not

unusual for them to be present on or in foodstuffs in low

concentrations. The permissible residue levels allowed in

foodstuffs intended for human consumption are closely regu-

lated by government. However, not all foods can be properly

inspected, so it is common for people to be routinely exposed

to small concentrations of these chemicals in their diet.

In addition, most animals cultivated in intensive ag-

ricultural systems, such as

feedlots

and factory farms, are

routinely treated with antibiotics in their feed. This is done

to prevent outbreaks of

communicable diseases

under

densely crowded conditions. Antibiotic use is especially com-

mon during the raising of chickens, turkeys, pigs, and cows.

Small residues of these chemicals remain in the meat, eggs,

milk, or other products of these animals, and are ingested

573

by human consumers. Also, growth hormones are given to

beef and dairy cows to increase their productivity. Small

residues of these chemicals also occur in products eaten by

consumers.

Strictly speaking, residues of pesticides, antibiotics,

and growth hormones are not additives because they are not

added directly to foodstuffs. Nevertheless, these chemicals

are present in foods eaten by people, and many consumers

find this to be objectionable. So-called organic foods are

cultivated without the use of synthetic pesticides, antibiotics,

or growth hormones, and many people prefer to eat these

foods instead of the much more abundantly available food-

stuffs that are typically sold in commercial outlets. (Note

that the term “organic foods” is somewhat of a misnomer,

because all foods are organic in nature. The phrase “organic”

in this sense is used to refer to foods containing additives

and/or residues, etc.)

Irradiation of food

Irradiation is a new technology that can be used to

prevent spoilage of foods by sterilizing most or all of the

microorganisms and insects that they may contain. This

process utilizes gamma radiation, and it is not known to

cause any chemical or physical changes in foodstuffs, other

than the intended benefit of killing organisms that can cause

spoilage. Although this process displaces some of the uses

of preservative chemicals as food additives, irradiation itself

is somewhat controversial. Even though there is no scientific

evidence that

food irradiation

poses tangible risks to con-

sumers, some people object to the use of this technology and

prefer not to consume foodstuffs processed in this manner.

[Bill Freedman Ph.D.]

ESOURCES

OOKS

British Nutrition Foundation. Why Additives? The Safety of Foods. London:

Forbes, 1977.

Freed, D. L. J. Health Hazards of Milk. London: Bailliere Tindall, 1984.

Marcus, A. I. Cancer from Beef: DES, Federal Food Regulation, and Consumer

Confidence. Baltimore: Johns Hopkins University Press, 1994.

Miller, M. Danger! Additives at Work: A Report on Food Additives, Their

Use and Control. London: London Food Commission, 1985.

Safety and Nutritional Adequacy of Irradiated Food. Geneva, Switzerland:

World Health Organization, 1994.

ERIODICALS

Etherton, T. D. “The Impact Of Biotechnology On Animal Agriculture

And The Consumer.” Nutrition Today 29, no. 4 (1994): 12–18.

Food and Drug Administration

Founded in 1927, the Food and Drug Administration

(FDA) is an agency of the Untied States

Public Health

Environmental Encyclopedia 3

Food chain/web

Service

. One of the nation’s oldest consumer protection

agencies, the FDA is charged with enforcing the Federal

Food, Drug, and Cosmetics Act, and other related public

health laws. The agency assesses risks to the public posed

by foods, drugs, and cosmetics, as well as medical devices,

blood, and medications such as insulin which are made from

living organisms. It also tests food samples for contaminants,

sets labeling standards, and monitors the public health effects

of drugs given to animals raised for food.

To carry out its mandate of consumer protection, the

FDA employs over 9,000 investigators, inspectors, and sci-

entists who collect domestic and imported product samples

for examination by FDA scientists. The FDA has the power

to remove from the market those foods, drugs,

chemicals

or medical devices it finds unsafe. The FDA often seeks

voluntary recall of the product by manufacturers, but the

agency can also stop sales and destroy products through

court action. About 3,000 products a year are found to be

unfit for consumers and are withdrawn from the marketplace

based on FDA action. Also, about 30,000 import shipments

each year are detained at the port of entry on FDA orders.

FDA scientists analyze samples of products to detect

contamination, or review test results submitted by companies

seeking agency approval for drugs, vaccines,

food additives

dyes, and medical devices. The FDA also operates the Na-

tional Center for Toxicological Research at Jefferson, Arkan-

sas, which conducts research to investigate the biological

effects of widely used chemicals. The Agency’s Engineering

and Analytical Center at Winchester, Massachusetts, tests

medical devices, radiation-emitting products, and radioac-

tive drugs. The Bureau of Radiological Health was formed

in 1971 to protect against unnecessary human exposure to

radiation from electronic products such as microwave ovens.

The FDA is one of several federal organizations that

oversees the safety of

biotechnology

, such as the industrial

use of

microorganisms

to processes waste and water

products.

In 1996, when the FDA declared that cigarettes and

smokeless

tobacco

are nicotine-delivery devices, it took re-

sponsibility for regulating those products under the authority

of the Federal Food, Drug, and Cosmetics Act. With regard

to these products, the FDA has issued federal mandates

concerning sales to minors, sales from vending machines,

and advertising campaigns.

[Linda Rehkopf]

ESOURCES

ERIODICALS

Gibbons, A. “Can David Kessler Revive the FDA?” Science 252 (April 12,

1991): 200–3.

574

Iglehart, J. K. “The Food and Drug Administration and Its Problems.”

New England Journal of Medicine 325 (July 18, 1991): 217–20.

RGANIZATIONS

U.S. Food and Drug Administration, 5600 Fishers Lane, Rockville, MD

USA 20857-0001 Toll Free: (888) INFO-FDA, <http://www.fda.gov>

Food chain/web

Food chains and food webs are methods of describing an

ecosystem

by describing how energy flows from one

spe-

cies

to another.

First proposed by the English zoologist Charles Elton

in 1927, food chains and food webs describe the successive

transfer of energy from plants to the animals that eat them,

and to the animals that eat those animals, and so on. A food

chain is a model for this process which assumes that the

transfer of energy within the community is relatively simple.

A food chain in a grassland ecosystem, for example, might

be: Insects eat grass, and mice eat insects, and fox eat mice.

But such an outline is not exactly accurate, and many more

species of plants and animals are actually involved in the

transfer of energy. Rodents often feed on both plants and

insects, and some animals, such as predatory birds, feed on

several kinds of rodents. This more complex description of

the way energy flows through an ecosystem is called a food

web. Food webs can be thought of as interconnected or

intersecting food chains.

The components of food chains and food webs are

producers, consumers, and

decomposers

. Plants and che-

mosynthetic bacteria are producers. They are also called

primary producers or autotrophs ("self-nourishing") because

they produce organic compounds from inorganic

chemicals

and outside sources of energy. The groups that eat these

plants are called primary consumers or herbivores. They have

adaptations that allow them to live on a purely vegetative

diet which is high in cellulose. They usually have teeth

modified for chewing and grinding; ruminants such as deer

and cattle have well-developed stomachs, and lagomorphs

such as rabbits have caeca which aid their digestion. Animals

that eat herbivores are called secondary consumers or primary

carnivores, and predators that eat these animals are called

tertiary consumers. Decomposers are the final link in the

energy flow

. They feed on dead organic matter, releasing

nutrients back into the ecosystem. Animals that eat dead

plant and animal matter are called scavengers, and plants

that do the same are known as saprophytes.

The components of food chains and food webs exist

at different stages in the transfer of energy through an ecosys-

tem. The position of every group of organisms obtaining

their food in the same manner is known as a

trophic level

The term comes from a Greek word meaning “nursing,” and

the implication is that each stage nourishes the next. The

Environmental Encyclopedia 3

Food chain/web

In an ecosystem, food chains become interconnected to form food webs. (Illustration by Hans & Cassidy.)

first tropic level consists of autotrophs, the second herbivores,

the third primary carnivores. At the final trophic level exists

what is often called the “top predator.” Organisms in the

same trophic level are not necessarily connected taxonomi-

cally; they are connected ecologically, by the fact they obtain

their energy in the same way. Their trophic level is deter-

mined by how many steps it is above the primary producer

level. Most organisms occupy only one trophic level; however

some may occupy two. Insectivorous plants like the venus

flytrap are both primary producers and carnivores. Horseflies

are another example: the females bite and draw blood, while

the males are strictly herbivores.

In 1942, Raymond Lindeman published a paper enti-

tled “The Tropic-Dynamic Aspect of Ecology.” Although

a young man and only recently graduated from Yale Univer-

sity, he revolutionized ecological thinking by describing eco-

systems in the terminology of energy transformation. He

used data from his studies of Cedar Bog Lake in Minnesota

to construct the first energy budget for an entire ecosystem.

He measured harvestable net production at three trophic

levels, primary producer, herbivore, and carnivore. He did

this by measuring gross production minus growth, reproduc-

575

tion,

respiration

, and excretion. He was able to calculate

the assimilation efficiency at each tropic level, and the effi-

ciency of energy transfers between each level. Lindeman’s

calculations are still widely regarded today, and his conclu-

sions are usually generalized by saying that the ecological

efficiency of energy transfers between trophic levels averages

about 10%.

Lindeman’s calculations and some basic laws about

physics reveal important truths about food chains, food webs,

and ecosystems in general. The First Law of Thermodynam-

ics states that energy cannot be created or destroyed; energy

input must equal energy output. The Second Law of Ther-

modynamics states that all physical processes proceed in such

a way that the availability of the energy involved decreases.

In other words, no transfer of energy is completely efficient.

Using the generalized 10% figure from Lindeman’s study,

a hypothetical ecosystem with 1,000 kcal of energy available

(net production) at the primary-producer level would mean

that only 100 kcal would be available to the herbivores at

the second trophic level, 10 kcal to the primary carnivores

at the third level, and 1 kcal to the secondary carnivores

at the fourth level. Thus, no matter how much energy is

Environmental Encyclopedia 3

Food irradiation

assimilated by the autotrophs at the first level of an ecosys-

tem, the eventual number of trophic levels is limited by the

laws which govern the transfer of energy. The number of

links in most natural food chains is four.

The relationships between trophic levels has some-

times been compared to a pyramid, with a broad base which

narrows to an apex. Trophic levels represent successively

narrowing sections of the pyramid. These pyramids can be

described in terms of the number of organisms at each tro-

phic level. This was first proposed by Charles Elton, who

observed that the number of plants usually exceeded the

number of herbivores, which in turn exceeded the number

of primary carnivores, and so on. Pyramids of number can

be inverted, particularly at the base; an example of this would

be the thousands of insects which might feed on a single

tree. The pyramid-like relationship between trophic levels

can also be expressed in terms of the accumulated weight

of all living matter, known as

biomass

. Although upper-

level consumers tend to be large, the population of organisms

at lower trophic levels are usually much higher, resulting in

a larger combined biomass. Pyramids of biomass are not

normally inverted, though they can be under certain condi-

tions. In aquatic ecosystems, the biomass of the primary

producers may be less than that of the primary consumers

because of the rate at which they are being consumed;

phyto-

plankton

can be eaten so rapidly that the biomass of

zoo-

plankton

and other herbivores are greater at any particular

time. The relationship between trophic levels can also de-

scribed in terms of energy, but pyramids of energy cannot

be inverted. There will always be more energy at the bottom

than there is at the top.

Humans are the top consumer in many ecosystems,

and they exert strong and sometimes damaging pressures on

food chains. For example,

overfishing

overhunting

can

cause a large drop in the number of animals, resulting in

changes in the food-web interrelationships. On the other

hand, overprotection of some animals like deer or moose

can be just as damaging. Another harmful influence is that

biomagnification

. Toxic chemicals such as

mercury

and

DDT released into the

environment

tend to become more

concentrated as they travel up the food chain. Some ecolo-

gists have proposed that the

stability

of ecosystems is associ-

ated with the complexity of the internal structure of the

food web and that ecosystems with a greater number of

interconnections are more stable. Although more studies

must be done to test this hypothesis, we do know that

food chains in constant environments tend to have a greater

number of species and more trophic links, whereas food

chains in unstable environments have fewer species and tro-

phic links.

[John Korstad and Douglas Smith]

576

ESOURCES

OOKS

Krebs, C. J. Ecology: The Experimental Analysis of Distribution and Abun-

dance. 3rd ed. New York: Harper & Row, 1985.

ERIODICALS

Hairston, N. G., F. E. Smith, and L. B. Slobodkin. “Community Structure,

Population Control, and Competition.” American Naturalist 94 (1960):

421–25.

Lindeman, R. L. “The Trophic-Dynamic Aspect of Ecology.” Ecology 23

(1942): 399–418.

Food irradiation

The treatment of food with

ionizing radiation

has been in

practice for nearly a century since the first irradiation process

patents were filed in 1905. Regular use of the technology

in food processing started in 1963 when the U.S.

Food and

Drug Administration

(FDA) approved the sale of irradiated

wheat and wheat flour. Today irradiation treatment is used

on a wide variety of food products and is regulated in the

United States by the FDA under a Department of Health

and Human Services regulation.

Irradiation of food has three main applications: exten-

sion of shelf life, elimination of insects, and the destruction

of bacteria and other pathogens that cause foodborne illness.

This final goal may have the most far-reaching implications

for Americans; the U.S. Centers for Disease Control (CDC)

estimate that 76 million Americans get sick, and 5,000 die

each year from illnesses caused by foodborne

microorgan-

isms

, such as E. coli, Salmonella, the botulism toxin, and

other pathogens responsible for food

poisoning

Irradiation technology involves exposing food to ioniz-

ing radiation. The radiation is generated from gamma rays

emitted by cobalt-60 or cesium-137, or from x rays or elec-

tron beams. The amount of radiation absorbed during irradi-

ation processing is measured in units called RADs (radiant

energy absorbed). One hundred RADs is equivalent to one

Gray (Gy). Depending on the food product being irradiated,

treatment can range from 0.05 to 30 kGy. A dosimeter, or

film badge, verifies the kGy dose. The ionizing radiation

displaces electrons in the food, which slows cell division and

kills bacteria and pests.

The irradiation process itself is relatively simple. Food

is packed in totes or containers, which are typically placed

on a conveyer belt. Beef and other foods that require refriger-

ation are loaded into insulated containers prior to treatment.

The belt transports the food bins through a lead-lined irradi-

ation cell or chamber, where they are exposed to the ionizing

radiation that kills the microorganisms. Several trips through

the chamber may be required for full irradiation. The length

of the treatment depends upon the food being processed