Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere.
This article examines air pollution, focusing on urban pollutants, incineration-generated pollutants, pollution from motor vehicles, pollution by incinerators, and indoor air pollution, with an emphasis on their implications for agriculture.
Sources of Urban Air Pollution Affecting Agriculture
Although the problems of air pollution have been recognized for many decades, they were once considered to be only of local significance, restricted to industrial urban areas.
With the current recognition of the destruction of stratospheric ozone, the greenhouse effect, worldwide forest destruction, and the acidification of lakes and coastal waters, air pollution assumes global significance, impacting agricultural productivity and soil health.
0Sources of urban air pollution include:
- Power generation
- Transportation
- Industry, manufacturing, and processing
- Residential heating
- Waste incineration
Except for waste incineration, all of these pollution sources depend on fossil fuel and, to a lesser degree, on fuel from renewable resources such as plant material. Therefore, all of them produce essentially the same pollutants, although the quantity of each substance may vary from source to source.
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Incineration-Generated Pollutants and Agricultural Concerns
The principal incineration-generated pollutants are carbon monoxide (CO), sulfur dioxide (SO2), a mixture of nitrogen oxides (NOx), a mixture of hydrocarbons referred to as volatile organic compounds (VOCs), suspended particulate matter (SPM) of varying sizes, and metals, mostly bound to particles.
Waste incineration also produces chlorinated dioxins and furans formed during the combustion of chlorine-containing organic substances, which can settle on crops and soil, affecting agricultural ecosystems.
Carbon Monoxide (CO) in Agricultural Environments
Most global emissions of this gas (60–90%) originate from natural sources, such as decomposition of organic matter and volcanic activities. The anthropogenic origin is primarily due to incomplete combustion of fossil fuel, particularly in internal combustion engines.
Thus, motor vehicles are the main culprits. Carbon monoxide is a colorless, odorless, highly toxic gas. Its toxicity is due to its ability to displace haemoglobin-bound oxygen. A lethal intoxication with CO can occur only in an enclosed space.
In open spaces, the effect of carbon monoxide is mitigated by dispersion, but it can still affect workers in agricultural settings near urban areas.
Sulfur Dioxide (SO2) and Its Effects on Crops
Sulfur dioxide is a colourless gas with a strong, suffocating odour, intensely irritating to eyes and the upper respiratory tract. Natural sources of sulfur dioxide are volcanoes and decaying organic matter.
Additionally, dimethyl sulfide from oceans is converted in the atmosphere to sulfur dioxide. Exposure to sulfur dioxide causes bronchial constriction and increases air-flow resistance, posing risks to agricultural workers with respiratory issues. It also damages plant leaves, reducing crop yields.
Nitrogen Oxides (NOx) and Soil Fertility
Nitric oxide (NO) is formed by natural processes such as lightning and microbial digestion of organic matter. Microbial digestion first produces nitrous oxide (N2O), which is then oxidized to NO. Anthropogenic formation of nitrogen oxides results from high-temperature combustion, whereby nitrogen in the air combines with oxygen.
Nitric oxide is readily oxidized in the atmosphere to NO2, and the mixture of both gases is referred to as NOx. The total amount of NOx formed during combustion and the ratio of NO to NO2 depend on the fuel-to-air ratio and the temperature of combustion.
Nitrogen dioxide is a reddish-brown, irritating, and extremely toxic gas. When inhaled, it causes inflammation of the lungs, which may develop into edema after a delay of several days.
A short exposure to 100 ppm is dangerous, and 200 ppm is lethal. NOx also contributes to soil acidification, affecting agricultural productivity.
Photochemical Smog and Crop Damage
The mixture of ozone, PAN (peroxyacetyl nitrates), and other by-products such as aldehydes and ketones creates a haze referred to as photochemical smog. Ozone is a respiratory toxin.
Because it has low water solubility, it penetrates deep into bronchioles and alveoli. Acute exposure to ozone, mostly an occupational hazard, damages respiratory tissue and causes edema, which may be fatal. Sublethal exposure increases sensitivity to bronchoconstrictive agents and infections.
Chronic exposure to ozone may lead to bronchitis and emphysema. Additionally, photochemical smog (i.e., ozone, PAN, and other by-products) is an irritant of mucous membranes, eyes, and skin, and it damages crops by reducing photosynthesis and growth.
Volatile Organic Compounds (VOCs) in Agricultural Settings
VOCs originate from both anthropogenic and natural sources. Natural sources include vegetation, microbial decomposition, forest fires, and natural gas. Anthropogenic emissions result from incomplete combustion of fossil fuels and from evaporation of liquid fuels and solvents during storage, refining, and handling, culminating in the formation of PAHs in the air.
At least 26 airborne PAHs, some potential carcinogens and mutagens, have been identified, posing risks to crops and agricultural workers.
Occupational Exposure and Food Chain Contamination in Agriculture
People in certain occupations, such as coke-oven workers and coal tar pitch workers, are at high risk. Their exposure may exceed that of the general population by a factor of 30,000 or more.
Additionally, urban-generated particles loaded with PAHs settle on land or water, and these carcinogens are likely to enter the food chain, affecting agricultural products and livestock.
Benzene, Ethylene, and Their Agricultural Implications
Benzene is a human bone marrow poison and a carcinogen implicated as a cause of myelocytic and acute nonlymphocytic leukemia.
Ethylene is one of the major products of automobile exhaust but may also be formed by other combustion processes. It contributes heavily to photochemical oxidants, which can harm crops and reduce agricultural yields.
Airborne Particles and Their Impact on Farming
Particles are referred to as suspended particulate matter (SPM). They may be divided into suspended solids and liquid droplets, both of which have effects on respiratory and systemic toxicity.
Natural sources of airborne particles include dust, sea spray, forest fires, and volcanoes, all of which can deposit on farmland, affecting soil and crop health.
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Metal Pollutants and Agricultural Health Risks
Among metal pollutants, lead, mercury, and beryllium are of special interest due to their toxicity. Lead may cause neurodevelopmental effects in children even at low levels. Other effects include cardiovascular, renal, gastrointestinal, haematological, and reproductive effects.
Mercury exposure can harm the brain, heart, kidneys, lungs, and immune system of people of all ages. Beryllium’s major toxic effects are pneumonitis (a disease characterized by lung inflammation) and berylliosis (a chronic pulmonary disease).
These metals can contaminate soil and water, posing risks to agricultural workers and crops.
Nonmetal Pollutants and Their Effects on Agriculture
Fluorides and asbestos are nonmetal pollutants. Fluorine, a by-product of coal combustion, is released entirely in the gas phase in relatively large quantities. Being a reactive element, it combines readily with other atoms and molecules to form fluorides, which are respiratory irritants.
Airborne asbestos originates from industrial use and the demolition of old buildings containing asbestos. Its health effects are mostly limited to asbestos workers and those incidentally exposed to it while performing their duties. These pollutants can settle on agricultural fields, affecting crop quality.
Motor Vehicle Pollution and Agricultural Impacts
In many cities around the world, motor vehicles are the principal source of NOx emissions. However, pollution-control devices perform satisfactorily only when properly maintained. Poor maintenance, tampering, and insufficient monitoring and inspection make the attainment of air quality standards problematic, impacting nearby agricultural areas.
Rubber Particle Pollution from Vehicles
Rubber particles from tires contribute to air pollution and water pollution as they are washed out with stormwater into the watershed, potentially contaminating agricultural water sources.
Incinerator Pollution and Its Agricultural Consequences
Another concern is the emission of airborne toxins by municipal and toxic waste incinerators. With the growing shortage of waste disposal sites and the increase in the cost of disposal, municipalities in and around the industrialized world are tending to dispose of municipal waste by incineration and to use the heat produced for energy generation.
Effectiveness of Incineration Facilities
Although modern incinerators may meet air quality standards for conventional pollutants, there is concern about the incineration of chlorine-containing compounds, such as bleached paper and poly(vinyl chloride) plastics, which produce toxic (and until recently, unregulatedynia) dioxins and furans that can contaminate agricultural land.
Chemical Waste and Its Agricultural Risks
Waste incinerators contribute significantly to air pollution by emitting toxic metals such as mercury, lead, zinc, cadmium, tin, and antimony. Reports indicate that people living in close vicinity to toxic waste incinerators had a greater prevalence of coughing, phlegm, wheezing, sore throat, eye irritation, emphysema, sinus trouble, and neurological diseases than those living some distance from incinerators, posing risks to agricultural communities.
Indoor Air Pollution in Agricultural Households
The main source of indoor air pollution in developing countries is the combustion of coal or biomass (wood, dung, agricultural waste, etc.) for heating and cooking in primitive, poorly vented stoves.
The pollutants in this case are respirable particles coated with PAHs, nitrogen dioxide, sulfur dioxide, carbon monoxide, and a variety of VOCs. Most of these pollutants are either irritants of tender tissues, respiratory and cardiovascular toxins, or both.
Additionally, some PAHs are carcinogens and mutagens, mostly affecting the respiratory system. In rural agricultural households, where women spend most of their time indoors performing household chores, exposure to fumes of biomass fuels might be the single most important health hazard for women.
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