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Criteria Air Pollutants

Major ambient air pollutants ("criteria pollutants") were first regulated by the federal government in 1970 with the establishment of National Ambient Air Quality Standards (NAAQS ).  These new regulations included both primary standards, which are designed to protect the public health and secondary standards, which are established to protect the public welfare. 

The specific criteria pollutants of concern were carbon monoxide (CO), sulfur dioxide (SO 2), total suspended particulate matter (TSP), nitrogen dioxide (NO 2), ozone (O 3), and lead (Pb).  Hereafter, the use of the term "standards" refers to all of the ambient concentration levels established for a given criteria pollutant. On July 1, 1987, PM10 (particulate matter with an aerodynamic diameter of 10 microns or less) standards were promulgated and in 1997, PM2.5 (particulate matter with an aerodynamic diameter of 2.5 microns or less) standards were also promulgated.

Particulate matter in general is an all-inclusive term referring to total suspended particulates, PM10 and PM2.5.  These pollutants are closely monitored via the Mecklenburg County ambient air monitoring network and the information that is gathered is used to help determine the status of our local air quality.

Ozone Ozone

Ground level ozone is the major component of photochemical smog. "Smog" is a generic term for the obtrusive brown haze that often hovers around the Charlotte-Metrolina area and approximately one hundred (100) other U S cities during the summer season.

Ozone is a severe lung irritant, which can initiate stinging of the eyes, choking and coughing during ozone episodes. Repeated exposures to high concentrations of ozone have been associated with respiratory problems, such as an increased rate of lung infections and permanent lung damage, in small children, the elderly, asthmatics and individuals with emphysema or other similar disorders.

In very high concentrations, even healthy adults experience a reduction in lung capacity when exposed for long periods or during vigorous outdoor exercise. Elevated ozone levels also impact crop yield. Some fruits and vegetables, particularly tobacco, grapes, soybeans and citrus fruits are highly sensitive to ozone.

The United States Environmental Protection Agency (EPA) estimates that annual crop damage caused by ozone amounts to $3 billion nationwide. Additional harmful effects are manifested on materials. Ozone destroys natural rubber very quickly requiring articles such as windshield wipers and weather stripping to be made with synthetic materials. It can also affect textile dyes in the same manner, as does ultraviolet radiation.

Carbon Monoxide (CO)
Carbon monoxide (CO) is produced by the incomplete combustion (i.e. the fuel is not completely burned during the combustion process) of fossil fuels in engines, boilers, furnaces, etc. It is a colorless, odorless gas that can pose a danger to people from localized concentrations found on traffic congested city streets. When inhaled, CO enters the bloodstream and reduces the body's ability to deliver oxygen to vital organs and tissues. 

At low concentrations, CO causes fatigue and impairs mental functions. The ill effects of excess CO exposure are especially serious for those who suffer from cardiovascular disease.  In higher concentrations, CO intoxication may actually result in death of the exposed individual(s). 

Local year round monitoring of CO began in 1976.  A violation would be recorded if there were more than one exceedance of the CO standard in a calendar year.  A violation of the carbon monoxide standard has not occurred since 1986 or even an exceedance of the standard since 1990 despite a steady growth in automobile registrations and number of vehicle-miles-traveled (VMT).  Less polluting engines found in newer vehicles is the main factor accounting for the reduction in CO concentrations in our air quality.  Mecklenburg County was officially designated as a carbon monoxide attainment area in 1995.

Sulfur dioxide (SO 2)
Sulfur dioxide (SO 2) can adversely affect public health primarily as a respiratory irritant, the environment by damaging crops and forming acid rain, and visibility reduction through the presence of suspended sulfate particulates in the atmosphere.  Monitoring for sulfur dioxide in Mecklenburg County began in the mid-1960s, but was discontinued in 1984 when ambient air concentrations were deemed to be at very low levels.  Monitoring resumed in 1994 and has continually demonstrated compliance with the annual, 3-hour, and 24-hour standards for sulfur dioxide levels.

Nitrogen dioxide (NO 2)
Nitrogen dioxide (NO 2) is primarily formed as a waste gas exhausted from incomplete fuel combustion and, like sulfur dioxide, can lead to crop damage and acid rain formation.  In a concentration as low as 0.5 ppm, nitrogen dioxide can begin to affect the respiratory system of children and asthmatics.  Monitoring for nitrogen dioxide began in the mid-1960s, but was discontinued in 1986 due to the presence of very low concentrations.  Monitoring was resumed in 1989 due to recent studies emphasizing the role of nitrogen dioxide in the formation of ozone. 

Nitrogen dioxide levels have remained steady and are not likely to exceed the NAAQS; however, new control strategies for limiting ozone formation will likely involve reducing nitrogen dioxide emissions from both industrial and mobile sources.

As we breathe, extremely small particulate matter (PM10) can easily be inhaled and penetrate deeply into the innermost recesses of our lungs.  Health effects from PM10 exposure depend on the type, amount, and duration of particles inhaled and vary widely from respiratory aggravation to the development of cancer.  PM10 monitoring results for Mecklenburg County indicate concentration levels consistently below the NAAQS.


Smaller-sized particles – those less than 10 micrometers in diameter – tend to pose the greatest health concern because they can pass through the nose and throat and get into the lungs. In an attempt to better protect the public's health, the US Environmental Protection Agency (EPA) determined that a more restrictive particulate matter standard was needed. 

The PM2.5 standard (15 micrograms of PM2.5 particulate matter/cubic meter of air/year and 65 micrograms of PM2.5 particulate matter/cubic meter of air/day) was adopted in 1997.  Mecklenburg County started monitoring for PM2.5 in 1999 and currently has three (3) sites in use.  The standard requires three (3) years worth of data to determine the area's compliance status. 

Compliance with the annual PM2.5 standard will be demonstrated when the three year average of the spatially averaged annual means is less than or equal to 15 micrograms per cubic meter. Only one of our monitoring sites has three (3) complete years of data at the present time. 

Currently all the data that we have collected demonstrates that when EPA makes designation in 2005, Mecklenburg County will be a PM2.5 non-attainment area.  The data even excludes the days when a forest fire in Western North Carolina caused our PM2.5 numbers to increase dramatically in 2000. 

On November 3, 2000, PM2.5 was recorded at 84.5 micrograms/cubic meter of air.  While on November 4, 2000, PM2.5 was recorded at 60.2 micrograms/cubic meter of air before the monitoring site was clogged.  EPA has allowed these two days to be exempt from determining annual compliance status because they were deemed as a "special event". 

Before data collection started for PM2.5, CO was thought to be the major pollutant of concern during the non-ozone season (October - March), but now the focus has changed.  PM2.5 data demonstrates that it is the major pollutant during the non-ozone season with significantly higher air quality index than CO.  

Currently, Mecklenburg County only forecasts the air quality during the ozone season. When EPA determines how to forecast the air quality index for PM2.5, it might be advantageous to provide forecasts throughout the entire year.

Lead (Pb) 
The NAAQS for Lead (Pb) was adopted in 1978.  Presently, it is set at 1.5 micrograms per cubic meter, maximum arithmetic mean over a calendar quarter.  Pb can be present in the air as either a particle or gas. 

Nationally in 1985, 73% of air-borne lead originated from motor vehicle combustion of gasoline containing anti-knock agents such as tetraethyl lead.  Essentially, there are no industrial sources of lead emissions in this area; virtually all local atmospheric lead emissions come from transportation sources.  In 1985, EPA mandates began reducing the lead content of gasoline. 

The standard for lead content in gasoline was 0.1 grams Pb/gal on January 1, 1986, but the complete prohibition of Pb from gasoline did not become effective until January 1, 1996.  Currently, Mecklenburg County is not conducting any ambient air lead sampling.

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