Important Terminology

Particulate Matter (PM)

Particulate matter (PM) refers to tiny solids or liquids (of various size) that are suspended in the air. Some particles are large enough to be seen with the naked eye (like dust, dirt, soot, or smoke). Others are so small they can only be detected using a microscope. Particles can irritate the eyes, nose, throat, and lungs causing decreased lung function and aggravated asthma. Long term exposure to them increases risk of heart disease, lung cancer, and chronic bronchitis. Particulate matter also impacts the environment by reducing visibility, affecting ecosystems (including plants, soil, and water), and contributing to climate change & acid rain.

Particulate matter is categorized by size and measured in micrometers (μm). For example, PM2.5 is smaller than PM10. To put these units into perspective, 1 micrometer (μm) is equivalent to 1/10,000^th of a centimeter.

PM₁₀ are particles that are less than or equal to 10 μm in diameter. These range from dust to pollen to mold. These particles are all about 5 to 7 times smaller in diameter than the average strand of hair.

PM_2.5 are particles that have diameters less than or equal to 2.5 μm. Their sources include industrial activities, vehicle emissions, wildfires, cooking and burning wood, and power plants. These particles are so small that they can penetrate deep into the lungs and bloodstream, leading to respiratory, cardiovascular, and immune system problems.

Volatile Organic Compounds (VOCs)

Volatile Organic Compounds (VOCs), are a group of chemicals that are emitted as gases from certain sources and can be found in paints, solvents, cleaning products, personal care items, new furniture, and carpets. The concentrations of many VOCs are consistently higher indoors (up to ten times higher) as compared to outdoor environments.

Most VOCs come from vehicle exhaust, industrial factory processes, consumer products, and natural emissions.

VOCs can have harmful effects. Short-term and long-term health effects include respiratory problems, eye & skin irritation, and cancer.

Carbon Dioxide (CO₂)

Carbon dioxide (CO₂) is produced in a variety of ways, including as a product of respiration but also from factories and industrialization processes. CO₂ is a greenhouse gas that absorbs heat radiating from the Earth’s surface, thereby accounting for the warming of the Earth.

High indoor CO₂ levels may indicate insufficient ventilation and impact the availability of oxygen that humans need to function properly.

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Relative Humidity (RH)

Relative humidity (RH) is the amount of water vapor that is present in the air as compared to the maximum amount of water vapor that the air can hold at a given temperature. RH is usually expressed as a percent.

Sources such as rain and bodies of water contribute to high outdoor humidity. Similarly, indoor contributors can range from boiling water on the stove to running a bath or shower.

Humidity can make the environment feel more moist, sticky, and typically increases how the temperature feels. Humidity can be measured with hygrometers, which use techniques such as electrical resistance, capacitance, or paired wet-bulb and dry-bulb temperature measurements to estimate the amount of water vapor in air.

Nitrogen Oxides (NO_x)

NO_x is the shorthand for nitric oxide (NO) and nitrogen dioxide (NO₂). These gases contribute to smog and acid rain. They are created when nitrogen and oxygen react during the combustion of fuels in the air.

High levels of NO_x can often be witnessed in large cities where there is a greater chance for motor vehicle traffic. This contributes to a significant source of air pollution.

NO_x also can be emitted naturally by lightning. However, combustion from vehicles is the most abundant source of NO_x.