40 ENVIRONMENTALENGINEERING
- LC50 and m50. Dose-response relationships for human health are usually
determined from health data or epidemiological studies. Human volunteers obviously
cannot be subjected to pollutant doses that produce major or lasting health effects,
let alone fatal doses. Toxicity can be determined, however, by subjecting nonhu-
man organisms to increasing doses of a pollutant until the organism dies. The LD50
is the dose that is lethal for 50% of the experimental animals used; LCso refers to
lethal concentration rather than lethal dose. LD50 values are most useful in compar-
ing toxicities, as for pesticides and agricultural chemicals; no direct extrapolation
is possible, either to humans or to any species other than that used for the LDso
determination. LD50 can sometimes be determined retrospectively when a large popu-
lation has been exposed accidentally, although estimates of dose in such cases is very
uncertain. - Homesis. Some substances appear to have a beneficial effect in small doses
and a detrimental effect when there is greater exposure. For example, soft (low-energy)
X-rays in very small doses are believed to stimulate the healing of broken bones. To
date, observations of hormesis have been rare and uncertain.
POPULATION RESPONSESIndividual responses to a particular pollutant may differ widely; dose-response
relationships differ from one individual to another. In particular, thresholds mer.
Threshold values in a population, however, generally follow a Gaussian distribution.
Individual responses and thresholds also depend on age, sex, and general state
of physical and emotional health. Healthy young adults are on the whole less sen-
sitive to pollutants than are elderly people, those who are chronically or acutely ill,
and children. Allowable releases of pollutants are, in theory, restricted to amounts
that ensure protection of the health of the entire population, including its most
sensitive members. In many cases, however, such protection would mean zero
release.
The levels of release actually allowed taking technical and economic control feasi-
bility into account. Even so, regulatory agencies try to set such levels below threshold
level for 95% or more of the U.S. population. For nonthreshold pollutants, how-
ever, no such determination can be made. In these instances, there is no release level
for which protection can be ensured for everyone, so a comparative risk analysis
is necessary. Carcinogens are all considered to be in this category of nonthreshold
pollutants.EXPOSURE AND LATENCYCharacterization of some health risks can take a very long time. Many cancers grow
very slowly and are noticed (expressed) many years, or even decades, after exposure to
the potentially responsible carcinogen. Current medical thought suggests that some car-
cinogens act by damaging a tumor-suppressing factor, and that expression occurs when