August 22, 1995

EPA-SAB-CASAC-LTR-95-004

Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M St., SW
Washington, DC  20460
                  Subject: CASAC Review of the Staff Paper for the Review of the
                  National Ambient Air Quality Standards for Nitrogen Dioxide:
                  Assessment of Scientific and Technical Information
Dear Ms. Browner:
      The Clean Air Scientific Advisory Committee (CASAC) of EPA's Science
Advisory Board (SAB) at a meeting on December 12, 1994, reviewed the document
entitled Review of the National Ambient Air Quality Standards for Nitrogen Dioxide:
Assessment of Scientific and Technical Information, Office of Air Quality Planning and
Standards (OAQPS) Staff Paper. At that meeting and in subsequent written comments,
the Committee made a number of recommendations for improving the document.  On
June 2, 1995, a revised Staff Paper was mailed to the CASAC members for review with
a letter response. The resulting comments by the Committee members note with
satisfaction the improvements made in the scientific quality and completeness of the
staff paper. It has been modified in accordance with the CASAC recommendations.

      The document is consistent with all  aspects of the scientific evidence presented
in the criteria document for oxides of nitrogen. It has organized the relevant
information in a logical fashion and the Committee believes that it provides a
scientifically adequate basis for regulatory decisions on nitrogen dioxide.  The staff
paper concludes, and the CASAC concurs, that an annual primary standard of the
present form and with a numerical value between 0.05 to 0.08 ppm would be supported
by the present scientific data on chronic health effects of exposure to nitrogen dioxide.
In addition, the CASAC concurs with the rationale that if the existing annual standard of
0.053 ppm is attained,  it will provide adequate protection against the occurrence of
short term 1-hour peak concentrations of 0.2 ppm or higher.

-------
      The staff paper also concludes and the CASAC concurs, that the present
primary standard protects against the direct effects of nitrogen oxide exposure to
vegetation. Further, the CASAC agrees that there is insufficient scientific evidence
available to warrant a secondary standard designed to protect aquatic and terrestrial
ecosystems from the adverse effects of nitrogen deposition including  eutrophication
and acidification.  Finally, CASAC concurs with the Staff conclusion that setting an N02
secondary standard to protect against visibility impairment is not warranted.

      The Committee looks forward to receiving notice of the revised or reaffirmed
nitrogen dioxide standard when it is proposed.

                                     Sincerely,
                                     Dr. George T. Wolff, Chair
                                     Clean Air Scientific Advisory
                                      Committee

-------