Water vapour has a profound infrared absorption spectrum with more and broader absorption bands than CO 2 , and also absorbs non-zero amounts of radiation in its low absorbing spectral regions,  (see greenhouse gas (GHG)), its GWP is therefore difficult to calculate. Further, its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16 °C, for example, creates an average humidity of ~18,000ppm at sea level (CO 2 is ~400ppm  and so concentrations of [H 2 O]/[CO 2 ] ~ 45x). Another issue with calculating GWP is that, unlike other GHG, water vapor does not decay in the environment, so an average over some time period or some other measure consistent with "time dependent decay," ., above, must be used in lieu of the time dependent decay of artificial or excess CO 2 , molecules. Other factors complicating its calculation are the Earth's temperature distribution, and the differing land masses in the Northern and Southern hemispheres.
PAHs are of concern as potential human health hazards, because many PAHs are demonstrated tumorigenic agents in animal bioassays and are positive in tests for genotoxicity or DNA damage. PAHs occur in the environment primarily as complex mixtures generated from the combustion of substances containing carbon and hydrogen, and rarely occur in the environment as isolated entities. The draft Relative Potency Factor approach is not a reassessment of individual PAH carcinogenicity, but rather provides a methodology for estimating cancer risk from exposure to PAH mixtures by summing doses of component PAHs after scaling the doses (with RPFs) relative to the potency of an index PAH (., benzo[a]pyrene). The cancer risk is then estimated using the dose-response curve for the index PAH.