Sum of the hazard quotients for substances that affect the same target organ or organ system.

Note: Ideally, hazard quotients should be combined for pollutants that cause adverse effects by the same mechanism. Aggregate exposures below a hazard index of 1.0 were unlikely to result in adverse health effects over a lifetime of exposure. A hazard index greater than 1.0 does not necessarily suggest a likelihood of adverse effects. The hazard index cannot be translated to a probability that adverse effects will occur, and is not likely to be proportional to risk.