Risky Business: PERCs
NAS Review of Perchloroethylene Risk Assessment Reveals Need
for Lower PEL
BY FRANKLIN MIRER
Perchloroethylene—also known as tetrachloroethylene, PCE, and
PERC, among other names—is the sweet smell in the dry cleaners.
PERC also contaminates ground water and enters homes via
vapor intrusion—and, likely, through evaporation from dry-cleaned clothing.
In 1995, the International Association for Research on Cancer
(IARC) classified PERC as “probably carcinogenic to humans.”
The IARC summary of human studies noted the existence of
evidence for consistently positive associations between exposure to tetrachloroethylene and the risks for oesophageal
and cervical cancer and non-Hodgkin’s lymphoma. These
associations appear unlikely to be due to chance, although
confounding cannot be excluded and the total numbers in
the cohort studies combined are relatively small.
Thirteen years later, in 2008, EPA published an updated draft
risk assessment for PERC on its Integrated Risk Information
System (IRIS). The National Academy of Sciences (NAS), at
EPA’s request, reviewed the draft in February 2010; URLs for
the full texts of both documents appear in the sidebar. The NAS
and EPA documents are evidence that PELs and TLV®s allow
dangerous exposures and should wake industrial hygienists to
the lack of protection for workers against PERC.
The NAS review provides a teachable moment for industrial
hygienists regarding current methods of risk assessment. Methods aside, industrial hygienists are called on to integrate work
exposures with community exposures, which requires knowledge of EPA reference values. An EPA reference concentration
(RfC) is an estimate of inhalation exposure to a substance that
is likely to be without harmful effects during a lifetime; an EPA
reference dose (RfD) is the maximum acceptable oral dose of a
toxic substance. In addition, the European Union’s REACH legislation requires chemical suppliers to calculate and publish Derived No Effect Levels (DNELs) by methods very similar to that
used by EPA to calculate RfC and RfD.
The NAS Review
The NAS committee agreed with EPA’s conclusion that PERC
was “likely to be a human carcinogen” according to existing
criteria. EPA had estimated “a range of inhalation unit risks of
2 x 10-6 to 2 x 10-5 per microgram per cubic meter” based on
extrapolation from leukemia in rats in a National Toxicology
Program (NTP) bioassay. At the top end of the estimate, this
equates to an 0.14 chance of cancer (after 24/7/365/70 expo-
sure) at 1 ppm. The NAS committee questioned EPA’s reliance
on the leukemia results, and estimated five-fold lower range of
risk for kidney cancer and eight-fold lower for liver cancer. Regardless of the extrapolation, 100 ppm (six hours per day, five
days per week) was an effect level for increased liver tumors in
mice in laboratory studies. The OSHA PEL for PERC is 100 ppm.
No extrapolation is needed for cancer risk.
Regarding neurological effects, the NAS committee opined
that EPA should extrapolate from different studies than those
EPA chose, which would relax its RfC from the recommended 2
ppb to somewhere between 6 and 50 ppb (not ppm).
EPA took decades to get to this point. In 1988 the agency
published its first IRIS assessment, with an oral RfD of 0.01
mg/kg/day and no RfC. In 2001, EPA started updating the risk
assessment on the IRIS system. But in 2007, EPA’s then-assistant
administrator for its Office of Research and Development,
George Gray, reportedly overruled his staff and contracted with
NAS (rather than the EPA Science Advisory Board) to review
the draft risk assessment, which was not public. Gray had come
to EPA from the Harvard Center for Risk Analysis, which is
considered an anti-regulatory think tank. NAS convened a
study committee, but work couldn’t begin until the draft risk
assessment was released in 2008. The first of four committee
meetings was held in November 2008. Now that the report has
been released, it has to cycle back through EPA.
PERC Reviews Outside of EPA
An NTP inhalation bioassay showing clear evidence for cancer
from PERC was released in 1986. The Japan Industrial Safety
Association published similar results in 1993.
In 1997, the Agency for Toxic Substances and Disease Registry (ATSDR) published a “minimal risk level of 40 ppb (in air)”
for PERC. This level has no legal force, but some state and local
public health agencies use it in regulations for dry cleaners.
Compared to the slow-moving EPA timeline, occupational
developments were glacial. The 100 ppm OSHA PEL for PERC is
nearly 50 years old. ACGIH® adopted a TLV of 200 ppm in 1946
and lowered it to 100 ppm in 1961, which OSHA adopted as the
PEL in 1972. ACGIH dropped the TLV to 50 ppm in 1982 and
then to 25 ppm in 1992 (with the notation, “confirmed animal
carcinogen with unknown relevance to humans”) where it remains today. The NIOSH Pocket Guide, which states that PERC
is a carcinogen, lists no recommended exposure limit.