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Air Pollution Basics

Major ambient pollutants include:


Ozone is a strong oxidant generated in nature by photochemical reactions involving sunlight, nitrogen dioxide, and volatile organic compounds (Brunekreef & Holgate, 2002; WHO, 2006). 

Ozone molecule
Four representations for ozone. UCAR (Randy Russell).

Due to its oxidizing nature, inhaling ozone may cause possible tissue damage and deprivation of vitamin, alongside reactions with inflammatory cells that could trigger respiratory inflammation. (Brunekreef & Holgate, 2002) The WHO guideline for ozone is 100 μg/m3 per 8-hour mean (WHO, 2006), while the EPA standard for ozone is 0.07 ppm (National Ambient Air Quality Standards for Ozone; Final Rule, 2015) (The conversion between ppm to μg/m3 is about 1960 for ozone under 25ºC and 1 atmosphere pressure) (The National Institute for Occupational Safety and Health, 2003).

Nitrogen Oxides

nitric oxidenitrogen dioxide
Four representations for nitric oxide (NO) and nitrogen dioxide (NO2). UCAR (Randy Russell).

Nitric oxide (NO) and nitrogen dioxide (NO2) are part of a larger family called nitrogen oxides (NOx). They have a variety of effects on the environment and health depending on their concentration and the presence of other compounds where they are found. The most prevalent of them in the breathable region of the atmosphere is NO2. NO2 is very versatile and causes chain reactions in the atmosphere in the presence of other compounds. When in the path of sunlight, it can break down to form ozone and NO (Australian National Pollutant Inventory, 2014). NO is also a toxic gas and can react with volatile organic compounds in the air to turn back into NO2. This chain reaction can happen various times and so it is often hard to determine the actual concentration of these materials as they vary a lot with weather conditions. NOx is highly correlated with the presence of other pollutants and so is often used as a surrogate for measures of concentration. A very serious consequence of the presence of these pollutants in the air is the formation of acid rain, which has harmful effects on the environment (EPA, 2017).

Additional information: Nitrogen Oxides (NOx), Why and How they are Controlled? (EPA), Oxides of Nitrogen (Australian national Pollution Inventory), Nitrogen Dioxide (NO2) Pollution (EPA)

Carbon monoxide

carbon monoxide
Four representations for carbin monoxide. UCAR (Randy Russell).

Carbon monoxide is a toxic gas made of carbon and oxygen molecules. Carbon monoxide is generated from incomplete burning process of material that have carbon such as oil, coal, and wood. (U.S department of Health administration)

The United States Department of labor occupational Safety and Health Administration (2002) states that carbon monoxide health effects depends on the level of exposure. Health effects due to low concentration exposure could include various symptoms such as headache, dizziness, shortness of breath, muscles cramping. However, effects due to high level exposure could cause psychiatric impairment such as Parkinsonism and dementia.

Carbon monoxide limits have been set by United States Environmental Protection Agency at 9 ppm on 8 hours average.


PM10 are particles smaller than 10 microns in diameter. Usually, particles that fall between 10 microns and 2.5 microns are categorized as coarse mode particles.

The current WHO guideline for PM10 is 20 μg/m3 for annual mean and 50 μg/m3 for a 24-hour mean (WHO, 2006).

PM10 and PM2.5. EPA.


PM2.5 are particles that have a diameter below 2.5 microns. Particles as such are also referred to as fine particles. Specifically, particles that are sized between 0.1 microns and 2.5 microns are categorized as accumulation mode particles, which are primarily anthropogenic from combustions (especially heavy-duty traffic). Due to their small size, accumulation mode particles can last for days and travel relatively long distances. Studies have shown that fine particulate matters generally cause more health effects and mortality than coarse mode particles.

The current WHO guideline values for PM2.5 is set to 10 μg/m3 for annual mean and 25 μg/m3 for a 24-hour mean (WHO, 2006). As of 2013, EPA updated the standards for PM2.5 to 12 μg/m3 and 35 μg/m3 for annual mean and 24-hour mean, respectively.


PM2.5 and UFP. http://now.tufts.edu/articles/big-road-blues-pollution-highways

Ultrafine Particles (UFP) are particles that are smaller than 100nm in diameter. Despite their small size, UFP actually make up 84% of total particle count in the air; and since they have a greater surface area than larger particles with the same mass, their surface chemistry is more complicated than that of larger particles. Therefore, they are more dangerous in terms of triggering cardiovascular morbidities and could potentially penetrate into blood circulation and even brain.

To date, the measurement of UFP is still difficult, and thus no clear standard for UFP is yet available both in U.S. and worldwide.

For further pollutant information, please visit EPA website. Concentration data for parts of the world can be viewed and downloaded at OpenAQ website.