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Stop Basing on 98th Percentile

Particulate Air Pollution: Stop Basing on 98th Percentile

January 9, 1997

Under existing regulations, compliance with the 24-hour particulate standard is based upon the 99th percentile, meaning that communities are allowed to exceed the standard for one full week every two years. The new standards would make this regime even less strict, basing compliance upon the 98th percentile meaning that communities could exceed the standard for two full weeks every year. There is no public health basis for using either the 98th or 99th percentile. Numerous studies clearly indicate that exposure for short periods of time cause dramatic health impacts. There is no justification for allowing communities to breath such polluted air for even one full week every year. The proposed regulations must be based upon a single exceedance of the standard. Our analysis of the AIRS database indicates that an additional 27.6 million people would be protected if standards were based upon a single exceedance rather than the 98th percentile.