formed to other HAP species, so the degree of health risk depends on the relative toxicity of the original HAP and its transformation products. A further breakdown of HAP transformations is given in Table 5.2, which shows that many of the 82 compounds that may react in the atmosphere, but for which no transformation product data were found, are oxygenated and nitrogenated organic compounds. Even for those chemicals with reported reaction products, the list of products may well be incomplete, depending on the rigor of the experimental study. For example, few studies have reported mass balances to document the completeness of the list of products. This lack of product data represents a serious gap in our knowledge about hazardous air pollutants and affects our ability to assess the health risks posed by their atmospheric transformations. Additional research is needed to elucidate the products and lifetimes of HAPs representing various classes on the HAPs list.