Bronte Wells
While there are many pollutants that are harmful to human health and the environment, some are considered more major than others. The Environmental Protection Agency (EPA) in the US, for example, does not consider carbon dioxide a major air pollutant when calculating the Air Quality Index (AQI). Similarly, methane, while a greenhouse gas, is often omitted from categories of pollutants because it is neither photoreactive nor toxic. Lead, which was once a major pollutant in gasoline, has been successfully removed from US fuels and is therefore no longer considered a major pollutant in emissions comparisons.
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What You'll Learn
Carbon dioxide
CO2 enters the atmosphere through the burning of fossil fuels such as coal, natural gas, and oil, as well as from solid waste, trees, and other biological materials. Certain industrial processes, such as cement production, also release CO2 through chemical reactions. The increase in CO2 emissions has led to the greenhouse effect, which has raised global temperatures and caused complex changes in weather patterns, known as global climate change.
To address the issue of rising CO2 levels and combat climate change, there have been efforts to transition to renewable energy sources and reduce carbon emissions. The Clean Air Act (CAA), established in 1970 and revised in 1977 and 1990, aimed to improve air quality and curb greenhouse gas emissions. However, there has been a long history of legal challenges to the implementation of these regulations.
Recently, there have been developments in legally defining carbon dioxide as a pollutant, giving the Environmental Protection Agency (EPA) more authority to regulate greenhouse gases and promote renewable energy sources. This new language has the potential to strengthen the EPA's ability to address climate change and block future legal challenges.
Despite these efforts, the United States, along with China, remains one of the largest producers of carbon dioxide emissions per capita. Initiatives such as the Clean Power Plan and the tightening of automotive fuel economy standards have been implemented to reduce carbon pollution and transportation emissions. While these initiatives are a step in the right direction, more comprehensive legislation and global cooperation are necessary to effectively combat climate change and reduce CO2 emissions.
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SOx
Sulfur oxides (SOx) are formed due to the presence of sulfur compounds in the fuel. The burning of fossil fuels by power plants and industrial facilities is the largest source of SOx in the atmosphere. Other sources of SOx emissions include industrial processes such as extracting metal from ore, natural sources like volcanoes, and vehicles that burn fuel with a high sulfur content.
While SOx emissions have been reduced in recent years, particularly with the use of ultra-low sulfur diesel, they remain a concern, especially for electric vehicles. Electricity generation is currently the largest source of SOx emissions. To address this issue, the EPA has implemented rules to reduce SO2 emissions and pollutants that form sulfur oxides, helping state and local governments meet national air quality standards.
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Lead
In 2006, ultra-low sulfur diesel regulations were implemented, and the removal of lead from motor vehicle gasoline in the US has resulted in a 98% decrease in lead levels in the air between 1980 and 2014. However, lead is still used in general-aviation gasoline for piston-engine aircraft, and the highest air concentrations of lead are typically found near lead smelters. Other sources of lead emissions include ore and metals processing, waste incinerators, utilities, and lead-acid battery manufacturers. Lead is also used in the manufacturing of many products, such as pigments, paints, solder, stained glass, lead crystal glassware, ammunition, ceramic glazes, jewellery, toys, some traditional cosmetics, and some traditional medicines.
The widespread use of lead has led to significant environmental contamination, human exposure, and public health issues worldwide. Lead exposure can affect multiple body systems, including the nervous, kidney, immune, reproductive, developmental, and cardiovascular systems. It is particularly harmful to young children, contributing to behavioural problems, learning deficits, lowered IQ, and adverse effects on the development of the central nervous system. In 2021, lead exposure was attributed to over 1.5 million deaths globally, primarily due to cardiovascular effects.
To address the health risks associated with lead exposure, the World Health Organization (WHO) has identified lead as one of the ten chemicals of major public health concern. WHO has published guidelines and resources to manage lead exposure and encourage the development of legally binding laws to control the use of lead in products such as paint. As of January 2024, 48% of countries have legally binding controls on lead paint. Additionally, the United States Environmental Protection Agency (EPA) has implemented regulations to reduce lead emissions and lower the acceptable lead concentration levels in the air.
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Oxygenated hydrocarbons
Indoor solid fuel combustion is a dominant source of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs). OPAHs are believed to be more toxic than PAHs. However, there is limited quantitative information on the emissions of OPAHs from solid fuel combustion.
A study was conducted to measure the emission factors of OPAHs (EF(OPAH)) for nine commonly used crop residues and five coals burned in typical residential stoves in rural China. The total EF(OPAH) ranged from 2.8 ± 0.2 to 8.1 ± 2.2 mg/kg for crop residues and from 0.043 to 71 mg/kg for various coals. 9-fluorenone was the most abundant specie. The EF(OPAH) for indoor crop residue burning was 1-2 orders of magnitude higher than that from open burning, and fuel properties and combustion conditions, such as moisture and combustion efficiency, affected the results.
To better understand the formation of pollutant emissions, a 13C labelling technique has been developed to identify the influence of local molecular structure on the formation of particulate matter (PM). This technique has been applied to trace the conversion of individual carbon atoms to PM in oxygenated and hydrocarbon molecules. It was found that the conversion to PM of different atoms within a molecule varies depending on the identity of their neighbouring moiety. Oxygen-containing functional groups were shown to influence the formation of particulates, partly by reducing the conversion to PM of carbon atoms adjacent to oxygen atoms.
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Fluorinated refrigerants
CFCs, which were previously used as refrigerants, have been phased out due to their contribution to ozone depletion in the upper atmosphere. The production and consumption of CFCs are now regulated under the Montreal Protocol, and they are being replaced by HFCs and hydrofluoroolefins (HFOs). HFCs have similar applications to CFCs but are less toxic, reactive, and flammable.
While HFCs have helped reduce ozone depletion, they still pose a significant environmental concern due to their high GWP. HFCs can remain in the atmosphere for long periods, contributing to the greenhouse effect and global warming. This has led to efforts to find non-fluorinated alternatives to HFCs and reduce their use.
One alternative is the use of HFOs, which have lower GWPs than HFCs. For example, HFO-1234yf is a non-ozone depleting refrigerant that has been adopted by automobile manufacturers since 2012 for use in light-duty vehicles. However, some HFOs can degrade atmospherically and form trifluoroacetic acid (TFA) as a persistent degradation product, which has negative environmental impacts.
Overall, while fluorinated refrigerants are not considered major pollutants in the same way as other well-known pollutants like carbon monoxide or nitrogen dioxide, they still contribute significantly to global warming and climate change. The transition to non-fluorinated alternatives is ongoing, and regulations such as the American Innovation and Manufacturing (AIM) Act and the EPA's SNAP program are helping to reduce the use of HFCs and promote safer alternatives.
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Frequently asked questions
Carbon dioxide is not considered a major air pollutant by the EPA when calculating AQI.
Oxygenated hydrocarbons like alcohols and aldehydes are not considered to be major air pollutants. Additionally, methane, while a greenhouse gas, is often omitted from categories of major air pollutants as it is neither photoreactive nor toxic.
Yes, non-criteria pollutants are not regulated by the CAA or EPA, but they can still be harmful to health and air quality. These include carbon monoxide (CO) and nitrogen oxides (NOx).
Lead was once a major pollutant, causing adverse health effects through ingestion or inhalation. However, due to regulatory efforts, levels of lead in the air have significantly decreased, and it is no longer considered a major concern.
