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Great progress has been made in achieving national air quality standards and updates periodically based on the latest science. One sign of this progress is that visible air pollution is less frequent and widespread than it was in the 1970s.

However, air pollution can be harmful even when it is not visible. Newer scientific studies have shown that some pollutants can harm public health and welfare even at very low levels. In recent years revised standards for five of the six common pollutants subject to national air quality standards. Govt. made the standards more protective because new, peer-reviewed scientific studies showed that existing standards were not adequate to protect public health and the environment.

Status of common pollutant problems in brief

Today, pollution levels in many areas exceed national air quality standards for at least one of common pollutants:

• Although levels of particle pollution and ground-level ozone pollution are substantially lower than in the past, levels are unhealthy in numerous areas of the country. Both pollutants are the result of emissions from diverse sources, and travel long distances and across state lines.
  
  Long- and short-term exposures to fine particle pollution, also known as fine particulate matter (PM_2.5), can cause premature death and harmful effects on the cardiovascular system, including increased hospital admissions and emergency department visits for heart attacks and strokes. Scientific evidence also links PM to harmful respiratory effects, including asthma attacks.
  
  Ozone can increase the frequency of asthma attacks, cause shortness of breath, aggravate lung diseases, and cause permanent damage to lungs through long-term exposure. Elevated ozone levels are linked to increases in hospitalizations, emergency room visits and premature death.
  
  Both pollutants cause environmental damage, and fine particles impair visibility.
  
  Fine particles can be emitted directly or formed from gaseous emissions including sulfur dioxide or nitrogen oxides. Ozone, a colorless gas, is created when emissions of nitrogen oxides and volatile organic compounds react.
   
• For unhealthy peak levels of sulfur dioxide and nitrogen dioxide, study is being done  to determine where and how often unhealthy peaks occur. Both pollutants cause multiple adverse respiratory effects including increased asthma symptoms. Both pollutants cause environmental damage, and are byproducts of fossil fuel combustion.
   
• Airborne lead pollution, a nationwide health concern before it was phased out lead in motor vehicle gasoline under Clean Air Act, now meets national air quality standards except in areas near certain large lead-emitting industrial facilities. Lead is associated with neurological effects in children, such as behavioral problems, learning deficits and lowered IQ, and high blood pressure and heart disease in adults.
   
• In brief: How to work  with states and communities to reduce toxic air pollution
• Sandards based on technology performance have been successful in achieving large reductions in national emissions of air toxics. In addition, due to reduction in  the benzene content in gasoline, and more  stringent emission standards for on-road and nonroad diesel and gasoline engine emissions have been  significantly reduced in  emissions of mobile source air toxics.
• Now it is required to do  residual risk assessments and technology reviews covering numerous regulated source categories to assess whether more protective air toxics standards are warranted. It is required to make  standards  appropriate. Additional residual risk assessments and technology reviews are currently needed.