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In Reply: While we concur with Mr Donnay that carbon monoxide has adverse effects on respiratory health in its own right, we disagree with his assertion that carbon monoxide confounds the association between ozone and asthma. Carbon monoxide in the urban environment is mainly produced by motor vehicle emissions, and it is often regarded as a surrogate for exposure to such other such emissions collectively. These motor vehicle emissions also contain not only nitric oxide, which becomes NO₂, but particulate emissions (including ultrafine particles), and many different organic and inorganic species. Because levels of all of these pollutants are usually strongly intercorrelated (which is not surprising considering their shared source-origin in urban areas), it has not always proved possible to statistically assign some measure of "responsibility" to them individually when a strong association has been shown to exist between them all and some adverse health outcome over time.

But . . . [Full Text of this Article]

George D. Thurston, ScD
Department of Environmental Medicine
New York University School of Medicine
New York, NY

David V. Bates, MD
Department of Medicine (emeritus)
University of British Columbia
Vancouver

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