The Clean Air Act requires the U.S. Environmental Protection Agency to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, commonly referred to as criteria pollutants. These pollutants are known to harm the environment and human health. At QuantAQ, we measure five of the six criteria pollutants as well as nitrogen oxide. Below, we describe what they are, where they come from, and what the NAAQS threshold is for each pollutant.

Carbon Monoxide (CO) is a colorless, odorless gas that is a byproduct of incomplete combustion. Common sources of CO include any vehicle or machine that burns fossil fuels, gas appliances, and gas space heaters. Ambient CO is generally not a health risk; however, CO can become problematic in confined spaces where a source is present. For more on the effects of CO, please visit the EPA's website.

The current NAAQS values for CO are:

Nitric oxide (NO) and nitrogen dioxide (NO2) are gases made of nitrogen and oxygen and are generated or released from a variety of natural and human-generated sources. NO is a colorless, flammable gas whereas NO2 is a red-orange gas that is harmful but not flammable. At high concentrations, it can be highly toxic and is a primary constituent of smog. NO is a key oxidizing agent and is highly reactive with other compounds. Nitrogen oxides (known as NOx) are produced naturally by volcanoes, oceans, biological decay, and lightning strikes. Major human-made sources include the emissions from vehicles and power generation.

NO is not a criteria pollutant, whereas NO2 is. The current NAAQS values for NO2 are:

Ozone (O3) is a secondary pollutant that is not directly emitted - but formed - via photochemical reactions with NOx and volatile organic compounds (VOCs). O3 reaches its peak values around mid-day when photochemical activity is highest and is likely to be worse in urban environments, or other environments where large quantities of VOCs are present.

Ozone can cause coughing, make it difficult to breathe deeply, inflame and damage the airways, and aggravate lung diseases such as asthma, emphysema, and chronic bronchitis. For more on the health effects of O3, please see the EPA's website.

The current NAAQS values for O3 are:

Sulfur dioxide (SO2) is a gaseous air pollutant that forms when sulfur-containing fuel (e.g., coal, oil, diesel, etc.) is burned as well as from natural sources such as volcanoes. SO2 can contribute to acid rain and can harm trees and plants by decreasing growth. Exposure to SO2 can harm the human respiratory system and can be especially problematic for those with asthma.

Thanks to the Clean Air Act, SO2 has decreased across the United States by more than 94% since 1980.

The current NAAQS values for SO2 are:

Particulate Matter (PM) is a mixture of solid particles and liquid droplets in the air. Particles can range in size from 1 nm to 100's of µm and are generated from a combination of primary sources (e.g., soot, dust, etc.) and secondary sources via chemical reactions with atmospheric oxidants and/or VOCs. Rather than count each individual particle (there are a lot of them), we measure the total mass of all particles that are smaller than a specific diameter. The EPA tracks and regulates PM2.5 (the mass per meter cubed of air with a diameter less than 2.5 µm) and PM10 (the mass per meter cubed of air with a diameter of less than 10 µm). Sometimes, we also track PM1 (the mass per meter cubed of air with a diameter of less than 1 µm) as well as Ultrafine Particles (UFPs), which is the total number of particles less than 100 nm.

PM2.5 are fine inhalable particles and are largely derived from combustion and secondary organic aerosol. Fine particles are the primary cause of reduced visibility in the United States and also cause negative health effects. Exposure to fine particles can affect your lungs and heart and has been linked to a variety of health problems.

PM10 includes all PM2.5 mass as well as the mass of all particles between 2.5 µm and 10 µm. The larger particles are typically emitted as primary pollutants and can be caused by dust from construction sites, wind-blown dust from open lands and dry lake beds, pollen, and industrial sources. It typically has a different chemical composition than PM2.5 and has different health effects.

The current NAAQS values for PM2.5 and PM10 are: