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Outdoor Air Pollutants

National Ambient Air Quality Standards

The Clean Air Act, as amended in 1990, requires EPA to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants:

  • Particle pollution (often referred to as particulate matter),

  • Ground-level ozone

  • Carbon monoxide

  • Sulfur oxides

  • Nitrogen oxides

  • Lead

These pollutants can harm your health and the environment, and cause property damage. EPA calls these pollutants "criteria" air pollutants because it regulates them by developing human health-based and/or environmentally-based criteria (science-based guidelines) for setting permissible levels. The set of limits based on human health is called primary standards.

Another set of limits intended to prevent environmental and property damage is called secondary standards.  Of the six pollutants, particle pollution and ground-level ozone are the most widespread health threats.  National Ambient Air Quality Standards are reviewed periodically by the EPA, with input from the Clean Air Scientific Advisory Committee (CASAC) and other stakeholder groups, and can be revised. 

Image of Ozone Pollution


What is a Nonattainment Area?

The Clean Air Act and Amendments of 1990 define a nonattainment area as a locality where air pollution levels persistently exceed National Ambient Air Quality Standards or a locality that contributes to ambient air quality in a nearby area that fails to meet standards.

Designating an area as nonattainment is a formal rulemaking process, and EPA normally takes this action only after air quality standards have been exceeded for several consecutive years. Nonattainment areas are given a classification based on the severity of the violation and the type of air quality standard they exceed.

Common Air Pollutants in our Region

Montgomery County's air quality planning falls within the Washington Metropolitan Council of Government's region air quality planning.  The Washington region is a non-attainment area for ozone and fine particulate matter (PM2.5)

The region has had to demonstrate, through the development of a State Implementation Plan, or SIP, a plan of action to improve air quality in the region so that it would attain the NAAQS by a certain set date.

Efforts made based on the particulate matter and ozone SIPS have shown positive results. In May 2013, the Council of Governments petitioned the EPA to no longer classify the region as non-attainment for particulate matter.  

State Implementation Plans



Ozone is a gas that occurs both in the Earth's upper atmosphere and at ground level. Ozone can be "good" or "bad" for people's health and for the environment, depending on its location in the atmosphere.

  • The stratospheric or "good" ozone protects life on Earth from the sun's harmful ultraviolet (UV) rays.

  • Ozone at ground level is a harmful pollutant and is designated as a “criteria” air pollutant by the EPA.


How is Ground Level Ozone Formed?

Ground level ozone is formed when intense sunlight interacts with oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Ozone forming NOx and VOCs come from many sources including vehicle and power plant emissions, lawn mowers and other fuel burning equipment, and vapors from gasoline, paints, and industrial processes.

Ozone pollution is a concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe. Many urban and suburban areas throughout the United States have high levels of ground level ozone. But many rural areas of the country are also subject to high ozone levels as winds carry emissions hundreds of miles away from their original sources. 

Health Concerns of Ozone

Breathing ozone, a primary component of urban smog, can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue. Children and those with pre-existing lung problems (such as asthma) are sensitive to the health effects of ozone. Even healthy adults involved in moderate or strenuous outdoor activities can experience the unhealthy effects of ozone.

Graphic of how ozone is formed.

What is the Current National Ambient Air Quality Standard for Ozone?

As 2008, the national ambient air quality standard for ozone is 0.075 ppm per 8-hours. 

The primary (public health based) ozone standard has been revised several times, from a one hour standard of .12 parts per million established in 1991, to an 8 hour standard of .84 ppm in 1997, to the current standard.. In each revision, the standard has been lowered, and has become more protective of public health. 

What Can You Do to Reduce Ozone Pollution?

To reduce ozone pollution and improve regional air quality:

  • Carpool, telecommute, or take mass transit to get to work

  • Limit driving and combine errands

  • Limit engine idling

  • Use environmentally safe paints and cleaning products whenever possible

  • Conserve energy at home, at work, and everywhere to reduce power demand. 

Power plants are one of the main sources of NOx, oxides of nitrogen, an ozone forming (precursor) pollutant. Reducing energy demand in your home will result in fewer ozone forming emissions from power plants.

On Air Quality Action Days, when ozone levels are forecast to be unhealthy, you should also

  • Refuel vehicles after dark, when emissions are less likely to produce ozone

  • Delay using gasoline-powered lawn and garden equipment, including mowers

  • Wait for a cooler day to use oil-based paints or switch to non-solvent or low VOC-based paints

  • Delay using household, workshop, and garden chemicals

Further Information


Particulate Matter

Particle pollution is a mixture of microscopic solids and liquid droplets suspended in air. This pollution, also known as particulate matter, is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen or mold spores). Unlike summertime ozone, particle pollution can occur year-round, and is linked to serious health problems, even at concentrations found in many cities.

Particles come in a wide range of sizes. Fine particles are less than 2.5 micrometers in diameter. 

Some fine particles can be emitted directly (think of smoke from a woodstove). But most are formed secondarily from complex atmospheric reactions of gases such as NOx and sulfur dioxide (SO2) that are emitted from power plants, industries, cars, buses and trucks.

How small is 2.5 micrometers?

About 1/30th the diameter of the average human hair - so small, you'd need an electron microscope to see them.



What is the Current National Ambient Air Quality Standard for PM2.5?

The Current NAAQS for PM2.5 is 12 ug/m3 annually, and 35 ug/m3 daily.

Where can I get Regional Information on Particulate Matter Pollution?

For information on PM2.5 levels in the region and to learn when PM2.5 can be harmful, visit Clean Air Partners which provides real time measurements from regional air quality monitors.

What Can You Do To Reduce Particulate Matter Pollution?

All of the measures you can take to reduce ozone pollution will also reduce the emissions of PM 2.5 and precursor chemicals.  In addition,

  • Reduce or eliminate fireplace and wood stove use

  • Avoid using gas-powered lawn and garden equipment

  • Avoid burning leaves, trash and other materials

  • Use household, workshop, and garden chemicals in ways that keep evaporation to a minimum, or try to delay using them when poor air quality is forecast.

  • Replace your car's air filter and oil regularly


Standby Electric Generator Air Standards

In addition to noise concerns caused by electric generators, be aware of the following air quality issues when purchasing or placing electric generators:

  • Locate electric generators to minimize exhaust emissions impacts to on-site occupants and all nearby neighbors

  • The Montgomery County Air Quality Ordinance, Chapter 3 of the Montgomery County Code, prohibits the discharge of any visible emission (exhaust smoke) from a generator into the atmosphere.

  • Give careful consideration to the choices of fuel types (diesel, natural gas, or LP gas) available for your specific generator. There are advantages and disadvantages for each type fuel. Diesel powered generators might be cheaper to operate, but they typically produce more visible emissions and require a large fuel storage tank. 

Planning and Permitting Standards

  • Site Plan requirements for installation of all new aboveground generators, condensing units, and fuel tanks.

  • Minimum setback requirements for generators