Page 396 - Environment: The Science Behind the Stories

100 that reach our bodies in very low concentrations. In research
with bisphenol A, a number of studies with lab animals have
Percentage of test population affected by dose 50 Linear dose-response curve relative to body mass than people would receive in the environ-
found unconventional dose-response curves.
Researchers generally give lab animals much higher doses

ment. This is so that the response is great enough to be meas-
ured and so that differences between the effects of small and
large doses are evident. Data from a range of doses give shape
to the dose-response curve. Once the data from animal tests

responses to still-lower doses from a hypothetically large popu-
LD 50 are plotted, researchers can extrapolate downward to estimate
0 lation of animals. This way, they can come up with an estimate
Low High of, say, what dose causes cancer in 1 mouse in 1 million. A sec-
Dose ond extrapolation is required to estimate the effect on humans,
(a) Linear dose-response curve with our greater body mass. Because these two extrapolations
stretch beyond the actual data obtained, they introduce uncer-
tainty into the interpretation of what doses are safe for people.
100
Mixes may be more than the sum
Percentage of test population affected by dose 50 Linear dose-response curve It is difficult enough to determine the impact of a single haz-
of their parts

ard, but the task becomes astronomically more difficult when
multiple hazards interact. Chemical substances, when mixed,
may act together in ways that cannot be predicted from the

other’s effects, cancel out each other’s effects, or multiply
Threshold
each other’s effects. Whole new types of impacts may arise
LD 50 effects of each in isolation. Mixed toxicants may sum each
0 when toxicants are mixed together. Interactive impacts that
Low High are greater than the simple sum of their constituent effects are
Dose
called synergistic effects.
(b) Dose-response curve with threshold With Florida’s alligators, lab experiments have indicated
that the DDT breakdown product DDE can either help cause
sex reversal or inhibit it, depending on the presence of other
100 chemicals. Mice exposed to a mixture of nitrate, atrazine, and
aldicarb have been found to show immune, hormone, and
Percentage of test population affected by dose 50 Nonlinear inverted of single hazards one at a time. In toxicology, the complex
nervous system effects that were not evident from exposure to
each of these chemicals alone.
Traditionally, environmental health has tackled effects
dose-response curve
experimental designs required to test interactions, and the sheer
number of chemical combinations, have meant that single-sub-
stance tests have received priority. This approach is changing,

LD 50 but the interactive effects of most chemicals are unknown. CHAPTER 14 • Envi R onm E n TA l H EA lTH A nd T o xi C ology
0 Endocrine disruption poses challenges
Low High
Dose for toxicology
(c) Unconventional dose-response curve
As today’s emerging understanding of endocrine disruption
Figure 14.16 Dose-response curves show that organisms’ leads toxicologists to question their assumptions, unconven-
responses to toxicants may sometimes be complex. In a clas- tional dose-response curves are presenting challenges for sci-
sic linear dose-response curve (a), the percentage of animals killed entists studying toxic substances and for policymakers trying
or otherwise affected by a substance rises with the dose. The point to set safety standards for them. Knowing the shape of a dose-
at which 50% of the animals are killed is labeled the lethal-dose-50, response curve is crucial if one is using it to predict responses
or LD . For some toxic substances, a threshold dose (b) exists, at doses below those that have been tested. Because so many
50
below which doses have no measurable effect. Some substances, novel synthetic chemicals exist in very low concentrations
in particular endocrine disruptors, show unconventional, nonlin- over wide areas, many scientists suspect that we may have
ear dose-response curves (c) that are U-shaped, J-shaped, or
inverted. underestimated the dangers of compounds that exert impacts
at low concentrations. 395

M14_WITH7428_05_SE_C14.indd 395 12/12/14 3:04 PM