TOXICOLOGY 1155
exposure, suggesting that summation exposure values best
represent potential health effects (Lange, 2002).
A short-term exposure limit (STEL) has also been estab-
lished for many chemicals. STELs are for 15-minute periods
with at least 2 hours of exposure below the PEL, as an exam-
ple, with no more than four exposure periods (STELs) occur-
ring per day. When applying STELs, the PEL should not be
exceeded when these values are included in the TWA. If there
is an exceedance of the PEL, appropriate personal protective
equipment is then required.
Exposure limit values (TLV-TWA) are established using
three general criteria. First, in order of importance, are epi-
demiological data. Occupational and in some cases environ-
mental epidemiology studies provide the most important
information on the hazards from a chemical. Since there
are different types of epidemiological studies, those of the
greatest strength, in order, are: cohort, case-control, cross-
sectional, and ecological. Next is animal experimentation in
identifying hazards, and last are case studies or reports. The
ACGIH publishes documentation summarizing the basis for
establishing and setting TLVs and is often useful as a general
reference. Another good reference that provides summary
information on chemicals is Hathaway et al. (1991).
Exposure levels are given in units of mgm^3 , ppm, and
fibers per cubic centimeter (fcc). In most cases these values
are for inhalation, but there are some listed for skin (e.g.,
decaborane).
Another value that is of importance to toxicologists in the
industrial environment is IDLH (immediately dangerous to life
and health). The problem with IDLH is that it has two differ-
ent definitions (NIOSHOSHAUSCGEPA, 1985). The Mine
Safety and Health Administration (MSHA) (30 CFR 11.3[t])defines IDLH as the concentration that will cause immediate
death of permanent injury. However, NIOSH, in the Pocket
Guide (1994; see Table 1), defines this as the maximum con-
centration where one can escape within 30 minutes without
irreversible health effects. So care must be taken when using
IDLH values, as each source has completely different criteria.DOSE-RESPONSEIn toxicity there exists an increased response to a chemical
with the chemical’s increasing concentration. This is known
as the “dose-response effect” and is fundamental to toxicol-
ogy. In general, it can be said that every chemical has a dose-
response effect. The response is any repeatable indicator or
measurement that is used to evaluate the response of an organ-
ism to the chemical. At some point the concentration becomes
high enough that the response is 100%. Figure 1 shows time
of exposure to various concentrations of the chemical sodium
bisulfate (Haarmann and Lange, 2000). As the concentration
of each chemical varies there is a reduction in root length after
a given period of time. In many cases the curve would appear
reversed, where there would be no inhibition at the lower con-
centrations and inhibition at the higher levels. However, here,
for the root length, which was for radish-seed elongation,
the highest length is at the lower concentration of chemical.
The shape of the dose-response curve can provide informa-
tion on the effect of a chemical, and data extracted from this
relationship is often used in risk-assessment analysis. LD 50
and related values are extracted from dose-response curves.
Different formulas can be used to obtain this information as
well (Thomulka et al., 1996).0.1 1 10 100 1000 10000
Sodium Bisulfate (ppm)020406080100120140Root Length (mm)FIGURE 1 Dose-response curve for sodium bisulfate in Lake Erie water (from
Haarmann and Lange, 2000; with permission from Parlar Scientific Publications).C020_003_r03.indd 1155C020_003_r03.indd 1155 11/18/2005 11:09:28 AM11/18/2005 11:09:28 AM