Federal Contaminated Site Risk Assessment In Canada Part I: Guidence on Human Health Preliminary Quantitive Risk Assessment (PQRA)
2.5.2 Exposure Frequency and Duration
Most assumptions concerning exposure frequency and duration are arbitrary in nature, being based on best professional judgment. While it is not the intent to question such professional judgment, a less arbitrary basis for these assumptions is desirable. For purposes of preliminary quantitative risk assessments, the frequency of site visits (days per year) and duration of such visits (hours per day) should be based on the guidance presented in Table 4 unless, in the opinion of the risk assessor, alternate assumptions are more defensible. Justification for alternate assumptions must be provided and fully referenced.
Table 4: Exposure Duration And Frequency Assumptions For Preliminary Quantitative Risk Assessments
| |
Agricultural Land |
Residential Land |
Commercial Land |
Industrial Land |
Construction Worker |
| Hours per day on site |
24 |
24 |
8 |
8 |
8 |
| Days per week on site |
7 |
7 |
5 |
5 |
5 |
| Weeks per year on site |
52 |
52 |
52 |
48 |
2 |
| Dermal exposure events per day |
1 |
1 |
1 |
1 |
1 |
| Meals of contaminated foods consumed per day |
1 |
1 |
1 |
1 |
NA1 |
| Life expectancy (years)for amortization of carcinogen exposures2 |
56/75 |
56/75 |
56/75 |
56/75 |
56/75 |
1 - Not applicable
2 - If cancer risks are estimated for adults only, the 56-year duration of adulthood (20 to 75 years, inclusive) should be used; if cancer risks are estimated on the basis of lifetime average daily intake, then average life expectancy of 75 years should be used.
2.5.3 Exposure Equations
The preferred exposure equations to be employed for a limited number of exposure pathways are presented in Table 5. Additional equations may also be included where the assessor determines that other exposure pathways beyond those listed in Table 5 are required. In those cases, the Problem Formulation section of the PQRA report should provide an adequate explanation of the need to include those additional pathways. The source of any additional equations must be fully referenced.
Inhalation exposures will be derived on the basis of the time spent in the contaminated environment (1.5 hours per day if outdoors; 22.5 hours per day if indoors; see Table 3). However, soil ingestion exposures are considered to be independent of the time spent outdoors. Although it is unlikely that ingested soil would be delivered as a single bolus dose, it is equally unlikely that intake would be distributed uniformly throughout the day. Therefore, for purposes of conservatism, 100% of the daily unintentional intake of contaminated soil should be assumed.
|
