[CPEO-BIF] Vapor intrusion and cumulative exposure

[Lenny Siegel <lsiegel@cpeo.org>]

One of the challenges of vapor intrusion investigations is the 
identification and evaluation of exposures to other sources of the 
contaminants of concern or even other compounds with similar impacts on 
the human body. Sometimes this is called "background," but that term 
means different things to different people.

Typically, those conducting the investigation identify compounds such as 
TCE in outdoor air or even in household chemicals. By showing that some 
fraction of exposure is not caused by the direct migration of subsurface 
contamination into homes and other structures, they sometimes argue that 
the responsible party (for the subsurface contamination) is less 
responsible for the consequences of indoor exposures. At some sites, 
they simply subtract "background" from measured indoor contaminant 
concentrations to calculate risk. However, those other exposures may 
intensify the health consequences of vapor intrusion exposures, so they 
should not be ignored.

EPA's 2001 health risk assessment concluded that people exposed to 
"background" levels of TCE or similar compounds are more likely to be 
affected by additional exposures to TCE than people who receive the same 
TCE exposures without the "background" exposure. The chemicals, or their 
metabolic products have an additive impact on organs such as the liver 
and kidneys,

There's an analogy. If you make $5,000 a year, you don't have to pay 
taxes on it. But if you earn $100,000 a year already and make an 
additional $5,000, you'll pay a good chunk of that additional $5,000 to 
the government.

So in setting a risk-based cleanup goal for TCE vapor intrusion in a 
home, it makes a difference that the residents may also be drinking 
TCE-contaminated water, even if the drinking water concentration is 
below the legally promulgated standard. It makes a difference if they 
are regularly exposed to household products with a similar impact on the 
body. It makes a difference if they are also breathing or drinking PCE 
or TCA, even in outdoor air.

There are two ways to deal with this problem.

First, site-specific risk assessments should consider cumulative 
exposures to all compounds with the same effect on the body, even if 
they are from sources other than vapor intrusion, and even if some of 
the exposures are beyond the jurisdiction or control of the regulatory 
agencies and responsible parties. That is, it may be necessary to 
address vapor intrusion that adds to unregulated exposures, even in 
situations where the vapor intrusion itself may constitute "acceptable 
risk."

Second, as long as regulators in the field tend to weigh separately each 
exposure - by medium, contaminant, and even source - against the 
pertinent health-based standard for that exposure, health 
based-standards may need to adjust for the additional risk. EPA proposed 
to do this in its 2001 health risk assessment for TCE.

As I have argued before, vapor intrusion studies should be based upon 
conceptual site models that consider all sources, all pathways, and all 
receptors, not only for the primary contaminant of concern, but for all 
chemicals that target the same body systems.

And health standards should be based upon real-world scenarios, not 
"laboratory conditions" that only consider isolated exposures.

Lenny


--
Lenny Siegel
Director, Center for Public Environmental Oversight
c/o PSC, 278-A Hope St., Mountain View, CA 94041
Voice: 650/961-8918 or 650/969-1545
Fax: 650/961-8918
http://www.cpeo.org


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