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EC number: 235-727-4
CAS number: 12626-81-2
Short description of key information on absorption rate:
Dermal absorption of lead through unabraded human skin is considered to be minimal (<0.1%) and thus absorption of inorganic lead compounds through the skin has previously been considered to be of less significance than absorption through the respiratory and gastrointestinal routes.
Lead is most easily taken up into the body through inhalation or
ingestion – dermal uptake makes a negligible contribution to systemic
lead levels. Once taken up into the body, lead is not
metabolized. However, lead will distribute to a variety of tissue
compartments such as blood, bone and soft tissues. The half-life of lead
in the body varies as a function of body compartment. Lead in blood has
a half life of 30 – 45 days – measurement of lead in blood thus provides
an integrated assessment of average lead exposure (via all routes) over
the preceding month. Lead is retained far longer in bones. Depending
upon bone type, the retention time of lead can vary between 8 and 30
years. Such lead can both serve as a source of endogenous lead exposure
and as a cumulative index of exposure over a time frame of years. Lead
excretion is primary via urinary and biliary excretion routes.
Discussion on absorption rate:
Human data are available and superced the animals studies that
have been conducted - one of which is described here. Detailed studies
on dermal uptake in humans are described in section 7.10.5. Dermal
absorption of lead through unabraded human skin is considered to be
minimal and thus absorption of inorganic lead compounds through the skin
has previously been considered to be of less significance than
absorption through the respiratory and gastrointestinal routes. The most
recent guideline-conformed in-vitro dermal absorption study (Toner and
Roper, 2005) has established absorption of lead to be less than 0.1%.
Other quantitative estimates of dermal absorption are limited in
reliability with the most rigorous study (Moore et. al. 1980) suggesting
uptake on the order of 0.01 – 0.18%. However, the data from many
published studies on this aspect largely lack compliance with current
guideline requirements, and their reliability and relevance for human
health risk assessment is questionable.
Animal studies serve to validate mechanistic
inferences derived from observational human studies. The majority of
information pertaining to lead toxicokinetics has been accurately
defined in humans of different ages and degrees of susceptibility to
lead toxicity. A number of toxicokinetic models have been developed to
predict the effects of external lead exposure upon internal or systemic
levels of lead. The Integrated Exposure Uptake Biokinetic (IEUBK) is now
widely applied to assess relationships between environmental lead
exposure and blood lead in children. Due to limitations in the ability
of the IEUBK model to assess the deposition and subsequent
remobilisation of lead from bone, use of the IEUBK model is generally
restrict to predict exposures in chidren six years of age or younger.
Physiologically-based pharmacokinetic models
(e. g. the O'Flaherty Model) have been developed to predict lead uptake
in humans of all ages but is most commonly applied in the assessment of
adult exposures. Both the O'Flaherty and IEUBK models are available as
computer simulation models and are discussed in greater detail in
IUCLID5 section 7.10.5.
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