Environmental Engineering Reference
Food supply pathway is another important exposure route. Lead is often bio-
accumulated or concentrated in plants or fishes, thereby exposing those humans
who are ingesting it. But this study is limited to calculations of exposure through
ingestion of groundwater only. The exposure rates for other pathways such as inges-
tion of soil, consumption of fish/vegetables, and dermal exposure are out of scope
of this study.
Dose-response relationship is a quantitative relationship that indicates the agent's
degree of toxicity to exposed species. The method involves derivation of occupa-
tional, clinical and epidemiological studies of the risk of that agent. Dose is normal-
ized as milligrams of substance or pathogen ingested, inhaled, or absorbed (in the
case of chemicals) through the skin per kilogram of body weight per day (mg/kg-
day). The goal of this exercise is to have a mathematical relationship between the
amount of a toxicant (lead in this case) to which a human being is exposed and the
risk of a hazardous outcome from that toxicant.
Lead is both a non-carcinogen, and a potential carcinogen, as explained in the
earlier part of this study. So, there will be two parts of this study.
Lead is only slowly excreted by the body. Small amounts of lead ingested over a
long time can slowly accumulate to reach harmful levels. Harmful effects may
therefore develop gradually without warning. Short-term exposure to high levels of
lead may also cause harm.
The dose-response effects for non-carcinogens follows an existence of a level
after which certain observable level of effects take place. The threshold value is
represented by Reference Dose, or RfD which is an assumed dose below which
Concentration (RfC) or an oral Reference Dose (RfD) for lead. However, WHO
guidelines state the upper limit for lead concentration in drinking water to be
0.01 mg/L.ADD is divided by RfD to get the Hazard Quotient (HQ). If HQ is
greater than 1, the exposed population is assumed to be at risk, else not.
There are several inconclusive epidemiological studies of exposed workers which
provided limited evidence of cancers of the kidney, stomach, and respiratory tract. The
U.S.EPAhasclassiiedleadinGroupB2:Probablehumancarcinogen(USEPA 2006 ).