In the context of this report, photochemical oxidants are understood to include ozone, nitrogen dioxide, and peroxyacylnitrates. Many other compounds have been proposed as components of photochemical air pollution but, as little information is available concerning their biological significance, these substances have not been discussed in this document. As nitrogen dioxide is an important air pollutant in its own right, it is the subject of a separate document (World Health Organization, 1977). Thus this report deals mainly with ozone and "oxidants" as measured by the neutral buffered potassium iodide method (NBKI).