Many government agencies and expert groups have estimated a dose-rate of perfluorooctanoate (PFOA) that would protect human health. Most of these evaluations are based on the same studies (whether of humans, laboratory animals, or both), and all note various uncertainties in our existing knowledge. Nonetheless, the values of these various, estimated, safe-doses vary widely, with some being more than 100,000 fold different. This sort of discrepancy invites scrutiny and explanation. Otherwise what is the lay public to make of this disparity? The Steering Committee of the Alliance for Risk Assessment (2022) called for scientists interested in attempting to understand and narrow these disparities. An advisory committee of nine scientists from four countries was selected from nominations received, and a subsequent invitation to scientists internationally led to the formation of three technical teams (for a total of 24 scientists from 8 countries). The teams reviewed relevant information and independently developed ranges for estimated PFOA safe doses. All three teams determined that the available epidemiologic information could not form a reliable basis for a PFOA safe dose-assessment in the absence of mechanistic data that are relevant for humans at serum concentrations seen in the general population. Based instead on dose-response data from five studies of PFOA-exposed laboratory animals, we estimated that PFOA dose-rates 10-70 ng/kg-day are protective of human health.

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening-level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil-gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16-year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil-gas screening levels of 30, 50, and 100 mu g/m3 and nonparametric Kaplan-Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are 30 mu g/m3 at any distance above a dissolved-phase hydrocarbon source. Dissolved-phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous-phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are 30 mu g/m3 for vertical screening distances of 13ft (4m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100ft (10 to 30m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.

Issues associated with vapour intrusion have been considered in Australia since the early 1990's; however, as there is no regulatory guidance, the sampling and assessment of vapour intrusion can be difficult. In addition, the need to collect sufficient data to enable an appropriate assessment of vapour intrusion, and resulting health risk has to be balanced against the limitations placed on the project by clients. Best practice guidance with respect to modelling, sampling and quantifying vapour intrusion is available from international sources; however, the reality of undertaking these assessments within such regulatory climates, as well as under client driven limitations, is challenging. Issues associated with the assessment of these sites are particularly evident when reviewing a wide range of petroleum related sites across Australia. The following paper provides a realistic review of 47 petroleum contaminated sites where vapour intrusion has been assessed on the basis of both modelling and the collection of site-specific data. The review presented identifies the key limitations on the data that have been collected on these sites, and whether these limitations are significant enough to affect the outcome and degree of uncertainty of the assessment. The review also evaluates whether the data collected are sufficiently robust to be critically assessed against current theories associated with the migration of petroleum hydrocarbon vapours in the subsurface and potential for vapour intrusion issues. The paper identifies the key aspects of vapour intrusion assessments that must be addressed, in any regulatory or client driven environment, to enable an adequate assessment of an issue where the ultimate goal is the protection of human health.