NAAQS Pollutants & Air Toxics

The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (40 CFR part 50) for pollutants considered harmful to public health and the environment. The Clean Air Act identifies two types of national ambient air quality standards. Primary standards provide public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

The EPA has set National Ambient Air Quality Standards for six principal pollutants, which are called "criteria" air pollutants. Periodically, the standards are reviewed and may be revised. The current standards are listed below. Units of measure for the standards are parts per million (ppm) by volume, parts per billion (ppb) by volume, and micrograms per cubic meter of air (µg/m3). 



Averaging Time



Carbon Monoxide (CO)


8 hours

9 ppm

Not to be exceeded more than once per year

1 hour

35 ppm

Lead (Pb)

primary and

Rolling 3 month average

0.15 μg/m3 (1)

Not to be exceeded

Nitrogen Dioxide (NO2)


1 hour

100 ppb

98th percentile of 1-hour daily maximum concentrations, averaged over 3 years

primary and

1 year

53 ppb (2)

Annual Mean

Ozone (O3)

primary and

8 hours

0.070 ppm (3)

Annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years

Particle Pollution (PM)



1 year

12.0 μg/m3

annual mean, averaged over 3 years


1 year

15.0 μg/m3

annual mean, averaged over 3 years

primary and

24 hours

35 μg/m3

98th percentile, averaged over 3 years


primary and

24 hours

150 μg/m3

Not to be exceeded more than once per year on average over 3 years

Sulfur Dioxide (SO2)


1 hour

75 ppb (4)

99th percentile of 1-hour daily maximum concentrations, averaged over 3 years


3 hours

0.5 ppm

Not to be exceeded more than once per year

(1) In areas designated nonattainment for the Pb standards prior to the promulgation of the current (2008) standards, and for which implementation plans to attain or maintain the current (2008) standards have not been submitted and approved, the previous standards (1.5 µg/m3 as a calendar quarter average) also remain in effect.

(2) The level of the annual NO2 standard is 0.053 ppm. It is shown here in terms of ppb for the purposes of clearer comparison to the 1-hour standard level.

(3) Final rule signed October 1, 2015, and effective December 28, 2015. The previous (2008) O3 standards additionally remain in effect in some areas. Revocation of the previous (2008) O3 standards and transitioning to the current (2015) standards will be addressed in the implementation rule for the current standards.

(4) The previous SO2 standards (0.14 ppm 24-hour and 0.03 ppm annual) will additionally remain in effect in certain areas: (1) any area for which it is not yet 1 year since the effective date of designation under the current (2010) standards, and (2)any area for which an implementation plan providing for attainment of the current (2010) standard has not been submitted and approved and which is designated nonattainment under the previous SO2 standards or is not meeting the requirements of a SIP call under the previous SO2 standards (40 CFR 50.4(3)).  A SIP call is an EPA action requiring a state to resubmit all or part of its State Implementation Plan to demonstrate attainment of the required NAAQS.




Ozone is a unique pollutant in that it is exclusively a secondary pollutant. This means that it is formed through a complex series of chemical reactions initiated by the release of a primary pollutant such as gasoline vapors. When a volatile organic compound, such as the VOC’s in gasoline vapors, reacts with nitrogen dioxide (N02) in the presence of sunlight, OZONE is formed. Therefore the level or concentration of ozone formed is influenced by many factors, including concentrations of NO2 and VOC’s in the area, the intensity of the sun and local weather conditions. Ozone is a colorless and odorless gas that can have adverse health effects.

The 1990 National Ambient Air Quality Standards (NAAQS) revisions recognized ozone as a potential health hazard and therefore set limits on the concentrations permissible in the atmosphere.


Ozone is the most prevalent chemical found in photo-chemical air pollution, or smog. Ozone is known to cause harmful effects in both newborn and unborn infants. Furthermore, ozone can be harmful on the elderly or other persons with respiratory problems. In addition, when compared to other air pollutants, ozone is found to cause the most damage to plants.

The effects of air pollution on crops, trees and other vegetation have been studied since 1970. Field studies and greenhouse experiments have shown that ozone is toxic to plants and can destroy a variety of commercial crops. Various cash crops such as oats, alfalfa, peas, and carrots are sensitive to acid rain, as are forested areas of the United States. There is also evidence that increased ultraviolet radiation caused by the loss of atmospheric ozone is affecting the normal growth cycles of plantlife.


One control strategy for ozone is to regulate sources of VOCs and nitrogen oxides. Major sources of these pollutants include the products of incomplete combustion of motor vehicle exhaust, the burning of fossil fuels, and the use of petroleum compounds and organic solvents in manufacturing and cleaning processes (i.e. one solvent used in dry cleaning is a VOC).


Although ozone is harmful at ground level (troposphere), it is a necessary component of the upper atmosphere (stratosphere). At this level, ozone acts to protect the layer of atmosphere which is responsible for filtering out harmful ultraviolet rays produced by the sun. So, although ozone is harmful and must be controlled in the troposphere, it is protective and must be protected in the stratosphere.

Carbon Monoxide



Carbon monoxide is a colorless and odorless gas that can be lethal in high concentrations. In nature, carbon monoxide is formed through the oxidation of methane, which is a common gas produced by the decomposition of organic matter. The primary man-made source of carbon monoxide is the incomplete combustion of fuels such as gasoline.For the combustion process to be complete, adequate oxygen must be present. When oxygen is not present in sufficient quantities, carbon monoxide is formed. The Clean Air Act of 1970 set the National Ambient Air Quality Standards (NAAQS), through which concentrations of carbon monoxide are limited to ensure public safety.


The health effects of carbon monoxide depend largely on the concentration of the compound found in ambient air and the duration of exposure to the higher concentrations. CO’s effect on humans results from the effect of carbon monoxide on the supply of oxygen in the blood stream. Normally, blood cells transport oxygen throughout the body. When carbon monoxide is present, the blood cells are more attracted to CO that to oxygen, resulting in oxygen deprivation. Short term exposure of high levels of CO can cause fatigue. Long term effects may influence a persons chances of developing lung and heart disease.


Such simple steps as tuning a vehicle’s engine can influence carbon monoxide levels. Well functioning mufflers can also help in lowering CO concentrations. Carbon monoxide is especially problematic in urban areas having a large number of automobiles. This is why many government agencies require vehicle inspections on a regular basis.

PM 2.5


What is PM2.5?

  • Particulate matter is the term used for a mixture of solid particles and liquid droplets found in the air. Coarse particles (larger than 2.5 micrometers) come from a variety of sources including windblown dust and grinding operations. Fine particles (less than 2.5 micrometers) often come from fuel combustion, power plants, and diesel buses and trucks.
  • These fine particles are so small that several thousand of them could fit on the period at the end of this sentence.
  • They are of health concern because they easily reach the deepest recesses of the lungs.
  • Batteries of scientific studies have linked particulate matter, especially fine particles (alone or in combination with other air pollutants), with a series of significant health problems, including:
    • Premature death; Respiratory related hospital admissions and emergency room visits; Aggravated asthma; Acute respiratory symptoms, including aggravated coughing and difficult or painful breathing; Chronic bronchitis; Decreased lung function that can be experienced as shortness of breath; and Work and school absences.

Effects of Fine Particles?

  • The Elderly:
    • Studies estimate that tens of thousands of elderly people die prematurely each year from exposure to ambient levels of fine particles.
    • Studies also indicate that exposure to fine particles is associated with thousands of hospital admissions each year. Many of these hospital admissions are elderly people suffering from lung or heart disease.
  • Individuals with Preexisting Heart or Lung Disease:
    • Breathing fine particles can also adversely affect individuals with heart disease, emphysema, and chronic bronchitis by causing additional medical treatment. Inhaling fine particulate matter has been attributed to increased hospital admissions, emergency room visits and premature death among sensitive populations.
  • Children:
    • The average adult breathes 13,000 liters of air per day; children breathe 50 percent more air per pound of body weight than adults.
    • Because children's respiratory systems are still developing, they are more susceptible to environmental threats than healthy adults.
    • Exposure to fine particles is associated with increased frequency of childhood illnesses, which are of concern both in the short run, and for the future development of healthy lungs in the affected children.
    • Fine particles are also associated with increased respiratory symptoms and reduced lung function in children, including symptoms such as aggravated coughing and difficulty or pain in breathing. These can result in school absences and limitations in normal childhood activities.
  • Asthmatics and Asthmatic Children:
    • More and more people are being diagnosed with asthma every year. Fourteen Americans die every day from asthma, a rate three times greater than just 20 years ago. Children make up 25 percent of the population, but comprise 40 percent of all asthma cases.
    • Breathing fine particles, alone or in combination with other pollutants, can aggravate asthma, causing greater use of medication and resulting in more medical treatment and hospital visits.

Control Efforts - A New Standard

  • Particulate matter originates from a variety of sources, including diesel trucks, power plants, wood stoves and industrial processes. The chemical and physical composition of these various particles vary widely. While individual particles cannot be seen with the naked eye, collectively they can appear as black soot, dust clouds, or grey hazes.
  • Those particles that are less than 2.5 micrometers in diameter are known as "fine" particles; those larger than 2.5 micrometers are known as "coarse" particles. Fine particles result from fuel combustion (from motor vehicles, power generation, industrial facilities), residential fireplaces and wood stoves. Fine particles can be formed in the atmosphere from gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds. Coarse particles are generally emitted from sources such as vehicles traveling on unpaved roads, materials handling, and crushing and grinding operations, and windblown dust.
  • EPA is also maintaining a national air quality standard focused on small particles less than 10 micrometers in diameter (known as "PM10") to protect against coarse particle effects. Ten micrometers are about one-seventh the diameter of a human hair.
  • In the years since the previous standard was enacted, hundreds of significant new scientific studies have been published on the health effects of particulate matter. Recent health effects studies suggest those adverse public health effects, such as premature deaths and increased morbidity in children and other sensitive populations, have been associated with exposure to particle levels well below those allowed by the current standard.




PM10 is suspended particulate matter in the air that when viewed under a microscope measures less than 10 microns in diameter. Particulate matter is formed by many processes including wind, the pollination of plants, and forest fires. The primary man-made sources of PM10 include the combustion of solid fuels such as wood and coal, agricultural activities such as fertilization and grain storage, and construction activities. PM10 was recognized as a criteria pollutant in 1987 (revised from the original total suspended particulate matter pollutant included in The Clean Air Act of 1970).


Particulate matter can have both health and welfare effects on humans. PM10 can contribute to increased respiratory illnesses such as bronchitis and can exacerbate the effects of other cardiovascular diseases. Visibility and the speed of deterioration of many man made materials can also be effected by particulate matter.


Many steps can be used to control PM10. Often businesses are required to treat gravel parking lots and alleys with chemicals designed to prevent the distribution of fugitive dust into the air. Furthermore control equipment may be required in various industries to impede the release of particulate matter produced as a by-product of their operations.



Nature and Sources of the Pollutant:

Nitrogen dioxide belongs to a family of highly reactive gases called nitrogen oxides (NOx). These gases form when fuel is burned at high temperatures, and come principally from motor vehicle exhaust and stationary sources such as electric utilities and industrial boilers.

A suffocating, brownish gas, nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone (or smog).

Health and Other Effects:

Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. The effects of short-term exposure are still unclear, but continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children. EPA's health-based national air quality standard for NO2 is 0.053 ppm (measured as an annual average).

Nitrogen oxides are important in forming ozone and may affect both terrestrial and aquatic ecosystems. Nitrogen oxides in the air are a potentially significant contributor to a number of environmental effects such as acid rain and eutrophication in coastal waters like the Chesapeake Bay.

Eutrophication occurs when a body of water suffers an increase in nutrients that reduce the amount of oxygen in the water, producing an environment that is destructive to fish and other animal life.


What are air toxics?

Air toxics are a group of air pollutants known or suspected to cause risk for serious health problems. Serious health effects include cancer and birth defects, as well as lung, immune system and nerve damage.

Air Toxic Source
Diesel Particulate Matter Diesel-fueled on-road and non-road mobile sources
Wood Smoke Particulate Matter The burning of wood, either in outdoor fires or indoor fireplaces and wood stoves Chromium Chrome electroplaters, combustion of distillate oil and on-road and non-road mobile sources Benzene On-road and non-road mobile sources, wood burning and other combustion
Carbon Tetrachloride Historically used as a solvent. No longer used commercially, but is a small by-product in sewage treatment plants Arsenic Combustion of distillate oil and combustion of chromium copper arsenate (CCA)-treated wood Chloroform Water treatment plants and reservoirs
1,3-Butadiene On-road and non-road mobile sources, wood burning and other combustion Acetaldehyde On-road and non-road mobile sources, wood burning and other combustion
Tetrachloroethylene (commonly known as ‘perc’) Dry cleaners
On-road mobile sources Includes cars, trucks, buses and motorcycles
Non-road mobile sources Includes cars, trucks, buses and motorcycles
Wood burning Includes burning wood indoors in fireplaces and wood stoves as well as outside (for example, land-clearing, yard debris or agricultural burning.)

How do air toxics impact health?

Did you know?
Adults breathe an average of 3,000 gallons of air a day.

Air toxics are associated with a broad range of health effects, including cancer. Because air toxics are primarily inhaled, many adverse health effects involve the respiratory system. Some studies have linked air toxics to asthma, a widespread respiratory disease. Other respiratory effects include lung inflammation, coughing, wheezing and reduced lung function. The cardiovascular, neurological, reproductive and immunological systems can also be affected by various air toxics.

People are affected differently by exposure to air toxics. Children, the elderly, pregnant women and those with compromised immune systems or illnesses are especially susceptible.

Some air toxics are persistent and bio-accumulative, meaning they build up in the food chain and result in higher exposures with relatively lower concentrations.

Local Air Toxics Monitoring
The Polk County Air Quality Division currently monitors for air toxics at the Carpenter Health Department monitoring site.



SO2 belongs to the family of sulfur oxide gases (SO2). These gases dissolve easily in water. Sulfur is prevalent in all raw materials, including crude oil, coal, and ore that contains common metals like aluminum, copper, zinc, lead, and iron. SO2 gases are formed when fuel containing sulfur, such as coal and oil, is burned, and when gasoline is extracted from oil, or metals are extracted from ore. SO2 dissolves in water vapor to form acid, and interacts with other gases and particles in the air to form sulfates and other products that can be harmful to people and their environment.

SO2 contributes to respiratory illness, particularly in children and the elderly, and aggravates existing heart and lung diseases.

SO2 contributes to the formation of acid rain, which:

  • damages trees, crops, historic buildings, and monuments; and
  • makes soils, lakes, and streams acidic

SO2 contributes to the formation of atmospheric particles that cause visibility impairment, most noticeably in national parks.

SO2 and the pollutants formed from SO2, such as sulfate particles, can be transported over long distances and deposited far from the point of origin. This means that problems with SO2 are not confined to areas where it is emitted.

High levels of SO2 over a short period, such as a day, can be particularly problematic for people with asthma. EPA encourages communities to learn about the types of industries in their communities and to work with local industrial facilities to address pollution control equipment failures or process upsets that could result in peak levels of SO2.



SO2 causes a wide variety of health and environmental impacts because of the way it reacts with other substances in the air. Particularly sensitive groups include people with asthma who are active outdoors and children, the elderly, and people with heart or lung disease.


Respiratory Effects from Gaseous SO2
Peak levels of SO2 in the air can cause temporary breathing difficulty for people with asthma who are active outdoors. Longer-term exposures to high levels of SO2 gas and particles cause respiratory illness and aggravate existing heart disease.

Respiratory Effects from Sulfate Particles
SO2 reacts with other chemicals in the air to form tiny sulfate particles. When these are breathed, they gather in the lungs and are associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death.

Visability Visibility Impairment
Haze occurs when light is scattered or absorbed by particles and gases in the air. Sulfate particles are the major cause of reduced visibility in many parts of the U.S., including our national parks.
Acidrain Acid Rain
SO2 and nitrogen oxides react with other substances in the air to form acids, which fall to earth as rain, fog, snow, or dry particles. Some may be carried by the wind for hundreds of miles.
Plant Water Plant and Water Damage
Acid rain damages forests and crops, changes the makeup of soil, and makes lakes and streams acidic and unsuitable for fish. Continued exposure over a long time changes the natural variety of plants and animals in an ecosystem.
Aesthetic Aesthetic Damage
SO2 accelerates the decay of building materials and paints, including irreplaceable monuments, statues, and sculptures that are part of our nation's cultural heritage.