Deferment of 24-Hour Average PM2.5 Standard
June 13, 2002

This page reviewed November 24, 2009.

There has been a change in the scope of the June 20, 2002 Public Hearing by the Air Resources Board to consider Amendments to the Ambient Air Quality Standards for Particulate Matter and Sulfates. Specifically, one aspect of the staff proposal (, regarding the establishment of
a 24-hour average PM2.5 standard, will not be considered at the Public Hearing.
Recent analyses have identified a problem in the application of a standard statistical software package often used in
time-series analyses over the past five years. Specifically, John Hopkins University reported a preliminary reanalysis for the National Morbidity, Mortality, and Air Pollution Study (NMMAPS) funded by the Health Effects Institute (see for more details).
After learning about the reanalysis of NMMAPS, ARB staff conducted an evaluation to determine how issues with NMMAPS and related studies might have affected our recommendations. We concluded that none of the long-term studies that support the staff recommendations for changes in the annual-average standard for PM10 and the establishment of a PM2.5 annual average standard used the statistical software package in question for estimates of PM-related premature death and increased disease. Accordingly, none of the recent information impacts the analysis of the proposed annual average standards and we think it is appropriate to go forward with these recommendations.
However, our evaluation found that some of the studies used as a basis for the recommended new 24-hour average PM2.5 standard did use the software in question. Therefore, we think that NMMAPS and other recent short-term studies of daily changes in death and disease need to be reanalyzed using a correct application of the statistical software package. Accordingly, it is prudent to take more time before recommending a specific level for a 24-hour average PM2.5 standard and, therefore, this aspect of the staff’s proposal will not be considered at Public Hearing.       
Staff will re-review the 24-hour-average PM standards and again go through the public review process when the studies in question have been reanalyzed.
The modified staff recommendations will be presented to the Board at a Public Hearing on June 20, 2002, in El Monte, California. Information on the hearing is available at To be considered by the Board, written comments not physically submitted at the hearing must be received no later than 12:00 p.m.,
June 19, 2002, submitted by the following methods: 1) email to, 2) mail to Clerk of the Board, Air Resources Board, 1001 "I" Street, 23rd Floor, Sacramento, CA 95814, or 3) by facsimile at (916) 322-3928.

Standards Review Schedule