Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p4
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 4/12)
Character Range: 1060073–1062963

action of the contaminant are clearly different for different contaminants.
    * ILCR estimates should only be summed where they relate to an exposed population that could plausibly be exposed to all of the contaminants / pathways that are added.
    * It is recognised that synergistic (that is, more than additive) effects are possible; however, the practical difficulties of quantifying the synergy in a contaminated land risk assessment are significant. Unless evidence for synergistic effects is available, the potential for synergistic effects may be omitted from the assessment. Additive effects are much more common and are covered in risk assessment by summing risks across chemicals and pathways.

6.4              Risk evaluation

    6.4.1          Threshold risk evaluation
The threshold HI assumes that there is a level of exposure below which it is unlikely for sensitive populations to experience health effects. If the exposure level does not exceed the threshold (that is, HI less than 1), then it is reasonable to conclude that no adverse health effects are likely to be realised. If the exposure level exceeds the threshold (that is, HI greater than 1) then further consideration is necessary. An HI greater than 1 does not automatically imply that an unacceptable risk is present but does indicate that further action(s) (investigation or risk management) is warranted. The HILs were developed on the basis of an HI of 1.

It is generally considered that the greater the HI, the greater the level of concern. HI ratios should not be interpreted as statistical probabilities; for example, an HI of 0.001 does not imply that there is a one in one thousand chance of the effect occurring (US EPA 1989).

    6.4.2          Non-threshold risk evaluation  acceptable level of cancer risk
Defining 'negligible risk' is a complex and potentially controversial issue. In debating this issue, consideration should also be given to the fact that people are routinely exposed to many naturally occurring carcinogens as a part of day-to-day living. Further, some people willingly expose themselves to carcinogens, for example, by smoking tobacco. However, people have the right to not be exposed to high risk agents artificially introduced into their environment. Furthermore, since the development of cancer is a step-by-step, cumulative process and people are exposed to many cancer-causing agents, the risk of any additional exposure should be negligible or set at a level that does not increase the existing risk by anything other than a negligible amount.

Historically, the life-time risk value of one in a 1,000,000 (often abbreviated as 1x10-6 or 10-6) was considered an insignificant risk and used as a maximum value for defining permissible environmental concentrations.

The US EPA has set a lifetime cancer risk goal of 1 in 1,000,000 for the regulation of individual