Outdoor Air Pollution: Natural Causes And Effects

Outdoor air pollution is a pressing issue that affects people in countries across income levels. It is a dynamic mixture of pollutants from both natural and manmade sources. Natural sources of outdoor air pollution include volcanic eruptions and wildfires, while manmade sources include burning fossil fuels, electric power generation, home heating, and motor vehicle transport. In 2019, outdoor air pollution was estimated to have caused 4.2 million premature deaths worldwide, with 89% of those occurring in low- and middle-income countries.

Wildfires, hurricanes, and other natural events

Outdoor air pollution is a pressing environmental health issue that affects people in low-, middle-, and high-income countries. It is estimated that in 2019, 4.2 million premature deaths worldwide were caused by outdoor air pollution, with 89% of those deaths occurring in low- and middle-income countries. The greatest number of deaths were in the WHO South-East Asia and Western Pacific Regions.

Hurricanes and tropical cyclones are influenced by air pollution in complex ways. While global warming, driven by greenhouse gas emissions, contributes to the increasing intensity of hurricanes, air pollution in the form of particulate matter has been shown to impact hurricane frequency. The interaction between these factors is evident in the contrasting effects of pollution on hurricane activity in different regions. For example, the reduction in pollution from North America and Europe has led to an increase in hurricanes in the Atlantic Ocean, while the growing emissions from India and China have reduced hurricane activity in the Western North Pacific.

It is important to recognize that most sources of outdoor air pollution are beyond the control of individuals, and addressing this issue requires concerted action from policymakers and decision-makers in sectors such as energy, transport, waste management, urban planning, and agriculture. The WHO Global Air Quality Guidelines provide valuable guidance on thresholds and targets for key air pollutants, aiming to reduce health risks and promote gradual improvements in air quality.

Volcanic eruptions

The eruption of Mount St. Helens in 1980 is a notable example of a volcanic event that resulted in international pollution. This eruption released up to 3750 tons of sulphur dioxide per day into the atmosphere, and the prevailing winds carried the ash cloud across the United States and around the world within 15 days. Another significant incident was the eruption of Mt. Kilauea in Hawaii in 1986, which released about 2,000 tonnes of sulphur dioxide into the lower troposphere, causing persistent acid rain and air pollution issues.

In addition to sulphur dioxide and sulphuric acid, volcanic eruptions also release carbon dioxide, a lightweight volatile gas easily expelled and dispersed during an eruption. Carbon dioxide's weight relative to air allows it to remain close to the Earth's surface, impacting the biosphere. High concentrations of carbon dioxide can be harmful to humans and other animal species, leading to unconsciousness and even death at levels above 30% inhalation.

Decomposing organic matter

The decomposition of organic matter can occur through aerobic and anaerobic processes. Aerobic decomposition takes place in the presence of oxygen, where living organisms feed upon the organic matter. This process is most common in nature and occurs on ground surfaces such as the forest floor, resulting in the formation of stable humus. Inadequate oxygen during aerobic decomposition can lead to the production of foul odours and slower degradation rates.

Anaerobic decomposition occurs in the absence or limited supply of oxygen. This process involves the collection of organic materials in pits, layered with soil, and left undisturbed for 6-8 months. Anaerobic conditions promote the growth of microorganisms that decompose organic matter and produce intermediates like hydrogen sulfide, methane, organic acids, and other substances. These compounds may accumulate and contribute to air pollution, especially in enclosed spaces.

The type of decomposition process and its rate are influenced by various factors. For example, the quality and quantity of organic matter, as well as the presence of certain microorganisms, play a role in determining the decomposition rate. Additionally, the degree of connectivity to the surface and cave depth can influence decomposition rates in subterranean ecosystems.

The gases released during the decomposition of organic matter, such as methane, contribute to outdoor air pollution. While natural, these emissions interact with human-made pollutants, such as vehicle emissions, fuel oils, and industrial fumes, to form complex mixtures that negatively impact human health and the environment.

Carbon monoxide from combustion

Carbon monoxide (CO) is a colourless, odourless, and toxic gas that is produced by the incomplete combustion of carbon-containing fuels such as natural gas, gasoline, charcoal, kerosene, or wood. It is emitted by a wide range of combustion sources, including motor vehicles, power plants, wildfires, and incinerators. While CO can be formed naturally from photochemical reactions in the atmosphere, human activities significantly contribute to its presence in the air.

Outdoor air pollution, caused by carbon monoxide and other pollutants, is a major environmental health problem. In 2019, it was estimated to have caused 4.2 million premature deaths worldwide, with 68% of those being due to ischaemic heart disease and stroke. Living in areas with high pollution levels can cause lung damage and increase the risk of developing asthma, bronchitis, and various cancers.

The sources of outdoor carbon monoxide pollution are primarily cars, trucks, and other vehicles or machinery that burn fossil fuels. Mobile sources, such as motor vehicles, are the most significant contributors to outdoor CO emissions, especially in urban areas. Additionally, natural sources of CO include wildfires and the release of methane and other hydrocarbons from organic matter in soils.

To address the health risks associated with carbon monoxide, the US Environmental Protection Agency (EPA) sets and reviews standards for CO levels in outdoor air under the Clean Air Act. These standards help state, tribal, and local agencies ensure that CO is maintained at a safe level. While indoor CO levels can be higher than outdoors, elevated outdoor CO levels can be concerning, especially for individuals with heart disease.

Reducing carbon monoxide emissions is crucial for mitigating the effects of global warming. As CO contributes to the formation of ozone, a climate change gas, strategies to reduce CO emissions are being considered as a potential approach to combat global warming.

Nitrogen dioxide from fuel combustion

Outdoor air pollution is a significant environmental health problem, causing an estimated 4.2 million premature deaths worldwide in 2019. One of the main contributors to this issue is nitrogen dioxide (NO2), a gas released during the combustion of fuels. NO2 is a major concern due to its impact on both human health and the environment.

Nitrogen dioxide is a highly reactive gas released during the combustion of fuels, particularly those with a significant nitrogen content, such as certain coals and oils. Transportation fuels, including gasoline, are a significant source of NO2 emissions, with car engines being a primary contributor. In areas with high motor vehicle traffic, such as large cities, nitrogen oxides emitted from vehicles can be a significant source of air pollution. Additionally, emissions from ships, aircraft, and other transportation sources also contribute to NO2 pollution.

Industrial processes and power plants are another major source of NO2 emissions. The combustion of fossil fuels, such as coal, gasoline, and natural gas, in factories, power plants, and incinerators, releases NO2 into the atmosphere. The formation of NO2 is highly temperature-dependent, with higher temperatures during fuel combustion leading to increased NO2 production.

The presence of NO2 in the atmosphere has significant health implications. It is known to irritate the human respiratory system and contribute to respiratory problems. Studies have linked exposure to NO2 with an increased risk of various health issues, including lung cancer, colorectal cancer, and prostate cancer. Children living near busy roads are more susceptible to developing asthma and bronchitis due to higher levels of air pollutants, including NO2.

In addition to its health impacts, NO2 also affects the environment. When excess nitrogen compounds are released into the atmosphere, they cause the acidification of soil, interfering with plant growth in both wild and agricultural contexts. Additionally, NO2 contributes to the formation of smog and acid rain, further exacerbating its environmental impact.

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