Guidelines for Metals and Metalloids in Ambient ... - ARCHIVE: Defra

Chromium
282. The WHO (2000) classifies Cr(VI) compounds as human carcinogens
by inhalation exposure. They were unable to recommend an air
quality guideline based on a safe level of exposure. A lifetime unit risk
for lung cancer of 4x10 -2 per µg/m 3 in air was derived by averaging the
results of studies of four cohorts of chromate production workers
(WHO, 2000). Extrapolation from this value by calculation equates
excess lifetime risks of 1 in 10,000, 1 in 100,000 and 1 in 1,000,000 with
exposures to concentrations of 2.5 ng/m 3 , 0.25 ng/m 3 or 0.025 ng/m 3 .
The WHO note that only a proportion of the total concentration of
chromium in air comprises Cr(VI) and that other forms of chromium
are not carcinogenic.
283. The USEPA Reference Dose (RfD) for chronic oral exposure to
Cr(III) is 1.5 mg/kg body weight day per day based on a no observed
adverse effect level (NOAEL) in a rat feeding study (USEPA, 1998).
This is an estimate of a daily exposure by ingestion that is likely to be
without an appreciable risk of deleterious effects during a lifetime. No
inhalation reference concentration (RfC) is available for Cr(III). The
US EPA RfD for noncancer effects arising from exposure to Cr(VI) is
3 µg/kg body weight day per day based on a NOAEL in a rat drinking
water study (USEPA, 1998). The RfC for non-cancer effects arising
from exposure to Cr(VI) in the form of chromic acid mists and
dissolved Cr(VI) aerosols is 8 ng/m 3 based on nasal septum atrophy in
exposed workers. This is an estimate of an airborne concentration that
is likely to be without an appreciable risk of deleterious effects during
a lifetime. The RfC for non-cancer effects arising from exposure to Cr
(VI) as particulate is 100 ng/m 3 based on the results of a 90 day study
in rats. The confidence of the USEPA in their RfCs for chromic acid
mists and Cr(VI) dusts are low and medium, respectively. The same
agency has estimated the unit risk factor for lifetime (lung) cancer risk
arising from exposure to Cr(VI) to be 1x10 -2 per µg/m 3 in air.
Concentrations of 8 ng/m 3 , 0.8 ng/m 3 and 0.08 ng/m 3 are extrapolated
to increased life-time cancer risks of 1:10 000, 1:100 000 and
1:1 000 000, respectively (USEPA, 1998).
284. The USEPA and WHO unit risks are of very similar magnitude, in
spite of being based on different occupational studies.
285. Defra and EA have not developed guidelines for inhalation exposures
to chromium, but have set an inhalation Index Dose to minimise the
risk of lung cancer arising from exposure to dust from chromium
contaminated soils. An Index Dose for inhalation exposure of 0.001
µg/kg body weight/day was derived for Cr(VI) which represents “a risk
level from possible exposure to a particular substance from a source”
and is accompanied by an ALARP (as low as reasonable practicable)
notation (also known as an inhalation Tolerable Daily Intake) (Defra
and EA, 2002). This is based on the calculated concentration of Cr(VI)
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