October 7, 1999 Office of the Chief Accountant Securities and Exchange Commission 450 Fifth Street, N.W. Washington, D.C. 20549 Dear Sir/Madam: We have read the second paragraph of Item 4 included in the Form 8-K dated October 7, 1999 of Thermatrix Inc. filed with the Securities and Exchange Commission and are in agreement with the statements contained therein. Very truly yours, /s/ Arthur Andersen LLP cc: Mr. Daniel S. Tedone, Executive Vice President and Chief Financial Officer, Thermatrix Inc.