Toxicology Excellence for Risk Assessment (TERA)

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Phone: 513-542-7475
Fax: 513-542-7487


Health Canada's Complex Exposure Tool (ComET) Responses to Post Meeting Questions

Questions received after the meeting are answered below. 

From Joshua Johnson of CDPR, Worker Health and Safety Branch, California .  How does the ComET address exposure when the chemical of question is not at equilibrium as is with many fumigants, and one that is clumped across the environment as with many herbicidal uses on rights-of-way?

The NearField algorithms do not necessarily assume equilbrium of release rates.

Conceptually ComET addresses nonequilibrium in the NearField as the daily average of the exposure incurred during any worst-case 24 hour period.   For instance if a gas were released from a product indoors in a 2 hour period and then stopped, the potential breathing zone concentration of that gas would be averaged over the entire 24 hour day for the estimate of acute exposure.  If this were the only day in the 91.25 day season then the average would be further amortized or averaged over this period for a sub chronic exposure estimate and if it were the only exposure during the year then it would be further averaged or amortized over 365 days for the estimate of chronic exposure. 

Your question, however, suggests a greater interest in environment or FarField sources.  Of course, we are still developing the details of this analysis scheme.  At this point the amount estimated to be released to the geographical volume would be assumed to be constant (well mixed) and at steady state.  Depending on how one assigns of the rate of this steady emission, this could result in a significant under or overestimation of exposure because of the basic assumption of equilibrium (constant emission to equilibrium).    Indeed, we recognize that acute and local exposures caused by concentrated releases in the FarField represent a major challenge for us to appropriately address them in the tool.


                                 2009 Toxicology Excellence for Risk Assessment