Environmental Toxicology- Study of Toxicants present In the Atmosphere

Environmental Toxicology deals with the effects of environmental toxicants on health as well as the environment. The word ‘health’ refers to that of both humans, animals, and plants as well as the other abiotic components of the environment.

Environmental toxicology is thus concerned with how environmental toxicants, through their interaction with humans, animals, and plants, influence the health and welfare of these organisms.

Environmental Toxicology

The second half of the twentieth century witnessed remarkable changes in the environmental perspective. The price of unchecked human progress through the first half of the century had led to a number of environmental catastrophes, which demonstrated the profoundly detrimental impact that such reckless prosperity could have on our natural environment.

These images of “modern” prosperity gradually gave rise to public outcry for governmental action to protect environmental quality, wildlife, as well as public health.

Environmental Toxicology: A specialized branch of Toxicology

The word ‘Toxicology’ is the study of the harmful effects of chemicals on biological systems. Humans, animals, and plants are increasingly being exposed to chemicals in the environment. As toxic chemicals are widespread in the environment, there is a potential for these chemicals to cause significant damage and harmful effects on human health.

In a broader sense, toxicology may involve activities such as the study of the detection, occurrence, properties, effects, and regulation of toxic substances. Hence, like medicine, toxicology is a multi-disciplinary subject covering many sub-disciplines including environmental toxicology. (Also read- Forensic Toxicology)

Environmental toxicology is concerned primarily with the movement and impact of toxicants and their metabolites in the environment, in food chains, and upon the structure and function of biological systems. The biological systems include any living systems such as humans and other mammals, plants, other organisms, and their habitats.

Environmental toxicology deals with incidental exposure of biologic tissue, and more specifically human tissue, to chemicals that are basically contaminants to the environment.

The Society of Environmental Toxicology and Chemistry (SETAC)

The Society of Environmental Toxicology and Chemistry is a non-profit organization with around 5300 members and institutions in over 90 countries, committed to the study, analysis, and solution of environmental problems, management, and regulation of natural resources.

The members have expertise in environmental toxicology and chemistry, biology, ecology, atmospheric sciences, health sciences, earth sciences, and environmental engineering, and are involved in environmental education, research, management, regulation, life-cycle, and risk assessment.

SETAC publishes papers describing original experimental or theoretical work that significantly advances understanding in the area of environmental toxicology, environmental chemistry, and hazard/risk assessment.

Nature Of Environmental Toxicants

Depending on their properties, these toxicants can produce their effect in different forms:

Contaminant Definition
Carcinogen Any substance capable of causing cancer in living tissue. Examples- tobacco, vinyl chloride(C2H3Cl), asbestos, dioxins(C4H4O2), etc.
  Teratogen    Any agent that disturbs the development of the growing embryo. Teratogens cause birth defects. Examples- alcohol, thalidomide-(C13H10N2O4 -a drug that was used to treat morning sickness during pregnancy), etc.
Mutagen Any agent capable of altering the genetic constitution of a cell by changing the structure of the hereditary material, DNA. Examples- radioactive substances, X-rays, UV rays, etc.
Neurotoxicant    Substances capable of causing adverse effects in the central and peripheral nervous system, and in sense organs as well. Examples- metallic contaminants like lead and mercury.
Endocrine Disruptor  Endocrine disruptors are chemicals that may interfere with the body’s endocrine system and produce adverse developmental, reproductive, neurological, and immune effects in both humans and wildlife. Examples- polychlorinated biphenyls(PCBs), pesticides like DDT(Dichlorodiphenyltrichloroethane) and BHC(Benzene hexachloride), plasticizers like bisphenol-A, etc.

Toxicants Of Air

The air we breathe started to get toxic since the time humans began to use wood fires for heat and cooking. Later, when soft coal was discovered and used for fuel, coal smoke became a problem in the cities. Increasing domestic and industrial combustion of coal caused air pollution to get steadily worse, particularly in large cities.

During the twentieth century, the most significant change was the rapid increase in the number of automobiles, which increased to millions within only a few decades.

In December 1952, an infamous killer London Smog occurred. Dense fog at ground level coupled with smoke from coal fireplaces caused severe smog lasting more than a week. The smog was so heavy that daylight visibility was only a few meters.

Two days after the smog began, the death rate began to climb, and between December 5 and December 9, there were an estimated 4000 deaths above the normal daily count. The chief causes of death were bronchitis, pneumonia, and associated respiratory complaints.

Types Of Air Toxicants (Pollutants)

What is clean air?

The true definition and composition of clean air or unpolluted air can be said to be unspecified as humans have been known to pollute the air since the advent of any human life on the earth. In addition, there are many natural toxicants of air such as terpenes from plants, smoke from forest fires, and fumes, and smoke from volcanoes.

Environmental Toxicology

Gaseous Toxicants

These are gases at normal temperature, and pressure and vapours evaporated from substances that are liquid or solid. Among pollutants of greatest concern are carbon monoxide (CO), hydrocarbons, hydrogen sulfide (H2S), oxides of nitrogen (NxOy), ozone (O3), sulfur oxides (SxOy), and CO2. Environmental toxicants, especially gaseous pollutants, are usually expressed as parts per million (ppm) by volume; in which 1 ppm = 1part pollutant per million parts (106).

Particulate Toxicants

These are fine solids or liquid droplets that can be suspended in the air. Some of the different types of particulates are defined as follows:

  • Dust: Relatively large particles about 100 μm in diameter that come directly from substances being used (e.g., coal dust, ash, sawdust, cement dust, grain dust).
  • Fume: Suspended solids less than 1 μm in diameter usually released from metallurgical or chemical processes, (e.g., zinc and lead oxides).
  • Mist: Liquid droplets suspended in the air with a diameter of fewer than 2.0 μm, (e.g., sulphuric acid mist).
  • Smoke: Solid particles (0.05–1.0 μm) resulting from incomplete combustion of fossil fuels.
  • Aerosol: Liquid or solid particles (<1.0 μm) suspended in air or in another gas.

Environmental Toxicology

Some Examples Of Air Toxicants: Environmental Toxicology

  • Carbon Monoxide: 

There are both natural and anthropogenic (resulting from the influence of human beings) sources of carbon monoxide, and atmospheric concentrations of this highly toxic gas are considerable. It is formed by the incomplete combustion of any fuel. The total world release of carbon monoxide from vehicles is of the order of hundreds of megatonnes.

It combines readily with haemoglobin (Hb) to form carboxyhemoglobin (COHb), thus preventing the transfer of oxygen to tissues. Haemoglobin is 210 times more prone to get combined with CO as compared to O2. A blood concentration of 5% COHb, equivalent to equilibration at approximately 45 ppm CO.

Concentrations of 100 ppm can cause headaches, dizziness, nausea, and breathing difficulties. An acute concentration of 1000 ppm is, without exception, fatal.

  • Sulfur Oxide:

Sulfur dioxide is the main oxide of sulfur that is readily formed in the atmosphere primarily from the industrial combustion of coal, with soft coal containing the highest levels of sulfur.


The sulfur oxides tend to adhere to air particles and enter the inner respiratory tract where it readily combines with water to form sulfurous acid resulting in the irritation of mucous membrane and bronchial constriction. This irritation, in turn, increases the sensitivity of the airway to other airborne toxicants.

  • Acid Rain:

Rain unaffected by sulfur dioxide naturally has a pH between 5.0 to 5.5 because of the carbon dioxide present in the atmosphere. With sulfur dioxide present in the atmosphere as a pollutant, the pH of the rain is further pushed to 4.5 which is sufficient enough to cause harmful effects to the aquatic life, soil biota, and cause necrosis of leaves of trees and also to erode monuments, Taj Mahal being the perfect example of the same.

  • Nitrogen Oxides:

Nitrogen oxides are produced in combustion processes, partly from nitrogen compounds in the fuel, but mostly by a direct combination of atmospheric oxygen and nitrogen in flames. Nitrogen oxides are produced naturally by lightning, and also, to a small extent, by microbial processes in soils.

Nitrogen dioxide is a pulmonary irritant and is known to lead to pulmonary oedema and haemorrhage. The main issue of concern is its contribution to the formation of photochemical smog and ozone, although nitrogen oxides also contribute to acid deposition.

  • Ozone:

A highly irritating and oxidizing gas is formed by the photochemical action of ultraviolet (UV) light on nitrogen dioxide in smog. The resulting ozone can produce pulmonary congestion, oedema, and haemorrhage.

NO2 + UV light → NO + O.

  O. + O2 → O3

At this point, it is worth distinguishing between “good” and “bad” ozone. Tropospheric ozone occurs from 0 to 10 miles above the earth’s surface and is harmful. Stratospheric ozone, located about 30 miles above the earth’s surface, is responsible for filtering out incoming UV radiation and thus is beneficial.

Location Ozone Level/ P.P.M
California(averaged over several counties and cities) 0.07
Shanghai, China 0.2
Delhi, India up to 0.1
Rural England (averaged over several sites) 0.03
Hong Kong up to .0.1

   Levels of ozone in different cities of the world

Volatile Organic Compounds

VOCs are released from burning fuel such as gasoline, wood, coal, or natural gas. They are also released from many consumer products like cigarettes, solvents, paints, etc.

Short-term exposure to various VOCs may cause irritation of the eyes and respiratory tract, headaches, dizziness, and memory problems. Long-term exposure to various VOCs can lead to loss of coordination damage to the liver, kidneys, and central nervous system, and even cancer.

  • Lead:

One of the most familiar of the particulates in air pollutants is lead. It is produced from burning fossil fuels such as coal, oil, gasoline, and natural gas; mining; and manufacturing. Lead and lead compounds are listed as “reasonably anticipated to be a human carcinogen.”

Lead can impair renal function, interfere with the development of red blood cells, and impair the nervous system, leading to mental retardation and even blindness. Exposure to high levels of lead for pregnant women can cause miscarriage, premature births, and smaller babies.

  • Solid Particulates:

The terms particulate matter (PM) or aerosol particles describe condensed (solid or liquid) material suspended in the atmosphere. They include crustal material, soot, combustion particles, nucleating clusters, and biological material such as spores, etc. The primary PM is released to the atmosphere from sources such as incomplete combustion, dust, and sea salt.

The secondary PM is formed in the atmosphere from the condensation of low volatility gases such as sulfuric acid, ammonia, and functionalized organic compounds. Pneumoconiosis, a condition common among coal miners that breathe coal dust, silicosis caused by breathing silica-containing dust, and asbestosis from asbestos fibers are all well-known industrial pollution diseases.

Atmospheric PM loading contributes to the reduction in visibility associated with poor air quality.

Pollutant Sources Significance
Sulfur Oxides, Particulates Coal & Oil Power Plants Oil refineries, smelters Kerosene heaters The main component of acid deposition.

Damage to vegetation& materials,

Irritation in lungs, Chronic Bronchitis


Nitrogen Oxide Automobile emissions

Fossil fuel power plants

Pulmonary Edema, Impairs Lung defenses,

Important Component of photochemical smog and acid deposition

Carbon Monoxide Motor vehicle emissions

Burning fossil fuels

Incomplete Combustion

Combines with haemoglobin to form carboxyhemoglobin, poisonous Asphyxia, and death

Asphyxia and death

Carbon dioxide Product of complete combustion May cause the greenhouse effect
Ozone(O3) Automobile emission

Photochemical Smog

Damage to vegetation,

Lung irritant

Hydrocarbon Smoke, Gasoline fumes,

Cigarette smoke, Industry,

Natural Sources

Contributes to photochemical smog,

Polycyclic aromatic hydrocarbons, lung cancer

Radon Natural Lung Cancer
Asbestos Asbestos Mines,

Building Materials,



Lung Cancer, mesothelioma


Allergens Pollen, House dust,

Animal dander

Asthma, rhinitis


Arsenic Copper Smelters Lung Cancer

Principal air toxicants, their sources, and effects

Environmental Effects

  • Vegetation:

Air toxicants may visibly injure plants by bleaching, necrosis, or by more subtle changes like alteration in growth and reproduction patterns.

Some examples are given in the table below:

Pollutants Symptoms
Sulfur dioxide Bleached Spots, Interveinal bleaching
Ozone Flecking, Stippling, Bleached spotting


Glazing, silvering, or bronzing on lower leaf surfaces
Nitrogen Dioxide  White or brown collapsed lesion near leaf margins
Hydrogen Flouride Tip and margin burns, dwarfing

Toxicants of air also cause measurable effects on forest ecosystems, such as the reduction in forest growth, change in forest species, and increased susceptibility to forest pests.

  • Domestic Animals:

Toxicants important in relation to toxicity in domestic animals are arsenic, lead, and molybdenum. Fluoride emissions from industries producing phosphate fertilizers and derivatives have damaged cattle throughout the world.

The raw material, phosphate rock, can contain up to 4% fluoride, some of which are released into the air and water. Farm animals, particularly cattle, sheep, and swine, are susceptible to fluorosis, which is characterized by mottled and soft teeth, and osterofluoritic bone lesions, which lead to lameness and, eventually, death.

  • Structures, Buildings, And Monuments:

Most of the buildings and structures, particularly those made of marble, in western Europe have been blackened and soiled due to smoke and damaged by chemical attacks from acid gases in the air. Metals are also affected by air pollutants; for example, SO2 causes many metals to corrode at a faster rate.

Ozone is known to oxidize rubber products. One of the effects of Los Angeles smog is the cracking of rubber tires. Fabrics, leather, and paper are also affected by SO2 and sulphuric acid, causing them to crack, become brittle, and tear more easily.

  • Atmospheric Effects:

The presence of suspended materials in the air can result in atmospheric haze or reduced visibility due to light scattering by the particles. Another major concern is the increase in the amount of carbon dioxide in the atmosphere which is responsible for causing the greenhouse effect, which further leads to global warming on the surface of the Earth.

In addition to CO2, other gases contributing to the greenhouse effect include methane, CFCs, nitrous oxide, and ozone.

  • Acid Deposition:

The acidic deposition is the combined total of wet and dry deposition, with wet, acidic deposition being commonly referred to as acid rain. Acid rain usually has a pH of less than 4.0. The low pH of the water bodies not only directly affects fish but also contributes to the release of potentially toxic metals, such as aluminium, from the soil.

Another area of concern is the reduced tree growth in forests. The leaching of nutrients from the soil by acid deposition may cause a reduction in future growth rates or changes in the type of trees to those able to survive in the altered environment.

A continuing trend toward urbanization of human populations has accompanied the industrialization of the world. Toxicity in the atmosphere by industry and by automobiles in the areas of concentrated populations has created a distinct public health hazard. The toxicants vary from gases to aerosols to dust, and from carbon monoxide to alkalis and acids.

Such contamination of the air exposes large populations to an unlimited variety of potentially harmful chemicals capable of producing known acute harmful effects and possible chronic debilitating illness in humans as well as the various other biotic components of the environment.

Conclusion: Role of Environmental Toxicology 

In the last three decades, Environmental Toxicology has attained much importance owing to the various health hazard risks the general population of humans, as well as animals, are being exposed to due to the irrational and maniacal use of chemicals in our daily lives. 

Environmental toxicants have been showing significant damage to the vulnerable population like pregnant and lactating women, geriatrics, clinical patients, etc. Anyone can be exposed to environmental toxicants via air, water, soil, and food.

Although, the sources of exposure to these toxicants can be miscellaneous. For example, heating food items in plastic containers lead to the leaching of chemicals from the plastic into the food. These chemicals are hazardous for health.

With the exposure of these chemicals to the environment, it has become more significant to study the trend in the environment and try to find the possible solutions that could solve the environmental issues.

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