The Difference between Vapor Intrusion Guidance and OSHA for Evaluating Indoor Air


The Fundamentals of Vapor Intrusion 

OHSA v VI Guidance

Many people believe that in the workplace, the maximum allowable concentrations of chemical vapors in indoor air are determined by the Occupational Safety and Health Administration (OSHA), regardless of the vapor source.  But subsurface chemicals entering the building through vapor intrusion are exceptions, and the differences can be confusing.

According to EPA’s 2002 Draft Vapor Intrusion Guidance, “EPA does not expect this guidance [sic] be used for settings that are primarily occupational”.  That could be interpreted to mean that OSHA always takes precedence in the workplace, but they continue to say, “However, employees and their employers may not be aware of subsurface contaminants that may be contributing to the indoor air environment of their workplaces, particularly since vapor intrusion may include constituents that are no longer or were never used in a particular workplace, may originate from elsewhere, or be modified by bio-degradation or other subsurface transformation processes.”  In a footnote, they added, “…there may be instances (under CERCLA and other cleanup programs) where standards other than the OSHA standards are used to determine whether the exposure pathway presents a risk to human health.”

All agree that vapors in the workplace emanating directly from a source within the workplace are subject to OSHA Permissible Exposure Limits (PELs).  The PEL is the time-weighted average concentration of a substance in air over eight hours, and it is a legally enforceable limit.  For example, a dry cleaner using tetrachloroethene (perchloroethene, PCE) can have a maximum average PCE concentration in indoor air of 100,000 parts per billion (ppb) under OSHA.  But if PCE enters the building as vapor intrusion, the indoor air screening level would be 27 ppb, according to EPA’s current Vapor Intrusion Screening Level (VISL) Calculator (commercial/industrial setting, 10-5 cancer risk, Hazard Index of 1).

Because EPA’s screening levels are based on EPA’s latest thinking on toxicity, they are revised more often than PELs, which are determined by legislative action.  Further complicating matters is that, unlike OSHA PELs, screening levels are applied differently from state to state, and sometimes within states.  For example, EPA’s VISLs would be determined for a 10-6 cancer risk in Illinois, 10-5 cancer risk in Ohio; and a 10-4 cancer risk under the Ohio Voluntary Action Program – but only in the workplace.   What’s more, OSHA PELs apply to individual chemicals without consideration of their additive effects, but vapor intrusion guidance normally requires determining the cumulative risk of multiple compounds.  This, and the fact that most OSHA PELs are based on decades-old data, results in OSHA PELs are typically orders of magnitude higher than VISLs.

What if vapors in indoor air are from both an indoor source and vapor intrusion?  That depends on the jurisdiction.  The Wisconsin Vapor Intrusion Guidance takes a pragmatic approach, and says that “When the contaminant of concern is also a chemical used in a manufacturing or commercial process, OSHA (Occupational Safety and Health Administration) standards or other occupational inhalation exposure guidelines apply to the indoor occupational exposure as long as the entity continues to use the chemical in question.”  But the Ohio Vapor Intrusion Guidance says that “For releases of contaminants to soil or groundwater that is the same as those used in on-site processes, a distinction must be made between the contribution to indoor air risk derived from the environmental media and the portion derived from on-site processes.”  Figuring out which PCE came from processes and which came from vapor intrusion is tricky, and arguably, pointless, in view of the potentially huge differences in concentrations and target levels.  But as they say, “Rules are rules.”

There’s also the issue of breakdown products.  EPA’s mention of bio-degradation reflects their concern that employees might be well informed about primary substances through labels, training, and Safety Data Sheets, but have little or no understanding of secondary compounds from vapor intrusion.  Employees could be exposed to harmful chemicals from vapor intrusion and never know it.  Trichloroethene (TCE) is a particular concern.  TCE is commonly used in industrial processes and household products, but it’s also a daughter product of PCE breakdown.  Some regulatory programs might let you ignore the intrusion of PCE if you’re using it in processes and staying below OSHA limits, but the associated TCE would only be exempt if it was also being used.

And finally, we’ve been asked about workplaces that don’t use and never used a chemical of interest.  If, for example, an office building is located near a dry cleaner, and PCE or TCE vapors intrude into the office space, the maximum allowable concentrations likely would be dictated by VISLs, not OSHA PELs.  A particularly sensitive situation would be a daycare or school, which may be a workplace, but which also has a sensitive population that is clearly not governed by OSHA limits.  The maximum allowable levels there would be VISLs, and likely residential target levels, which are several times lower than commercial/industrial levels.

So while the approach varies from place to place, one should never assume that vapor intrusion problems go away under OSHA.  As discussed elsewhere in this month’s Focus on the Environment, OSHA recognizes that its PELs are “inadequate for ensuring the protection of worker health”.  Many PELs will undoubtedly be lowered, but reducing them to vapor intrusion target levels would be a de facto ban on many substances.