supplies. However, there is uncertainty about what concentrations of these substances represent hazardous levels. One problem is that individuals and different
age groups may have varying tolerances to these substances; the other problem is our
present analytical methods. In some cases we do not have sufficient precision to
measure concentration in ranges low enough to detect their presence, whereas in
other cases we are improving our techniques by finding even lower concentrations
that may or may not prove to be harmful. To add to this dilemma are the present
requirements that in some instances tend to add to the problem of toxic materials. A
typical example is the requirement for chlorination of wastewaters before discharge
and of drinking waters prior to treatment. Chlorination of certain organic materials
produces the trihalomethanes, which are on the list of priority pollutants. If chlorination of wastewater is not practiced, there will be fewer trihalomethanes produced.
However, the downstream area will have a higher disease potential because the
pathogenic organisms have not been reduced sufficiently. Similarly, in the water
intake, where poor-quality water is used as the drinking water supply,
prechlorination has been practiced to reduce the number of pathogenic organisms.
This can result in an increase in the trihalomethane content. In order to reduce
trihalomethane production, it is recommended that prechlorination not be applied,
but that secondary chlorination, after the removal of the organics, be the only point
for chlorination. This of course will reduce the production of the trihalomethanes,
but may increase the potential for the carryover of pathogenic organisms in the water
supply system. Similarly, several heavy metals are on the list of priority pollutants.
Some of these metals have been shown to be essential growth factors in very low
concentrations. However, at only slightly higher concentrations, they have been
found to be detrimental. Thus, very close tolerance limits must be set, and it is
difficult to establish safe levels and still prevent the occurrence of unsafe levels.
Another problem comes in the determination of the priority pollutants. The
regulations call for the banning of any substance suspected of being carcinogenic
or mutagenic. This usually is based upon feeding massive doses to a susceptible
species of mice or rats. If any carcinogenic or mutagenic effects are found, that
substance is placed on the priority pollutant list. There is, however, no guarantee that
what is harmful to rats and mice will also be harmful to humans. Also it is difficult to
extrapolate to humans the concentration limits that were harmful to the animals.
A regulatory agency must of necessity be extremely conservative. Therefore, any
substance even suspected of being harmful must be included on this list. Environmental conditions are to be maintained so that even the most susceptible individual is
protected from any harm. Thus, the most conservative values and the lowest levels
are considered in listing the priority pollutants and their concentration limits.
One of the major problems in the evaluation of priority pollutants is the impact of
long-term use of any substance. There may be no immediate symptoms, but symptoms may occur after long periods of time, in another generation, or in a very indirect
manner. For example, the use of certain drugs (thalidomide) for increased fertility
has been shown to have an impact on the offspring of the second generation of
persons who used these drugs. Findings such as these take 20–25 yr to evaluate.
288
D. B. Aulenbach et al.
age groups may have varying tolerances to these substances; the other problem is our
present analytical methods. In some cases we do not have sufficient precision to
measure concentration in ranges low enough to detect their presence, whereas in
other cases we are improving our techniques by finding even lower concentrations
that may or may not prove to be harmful. To add to this dilemma are the present
requirements that in some instances tend to add to the problem of toxic materials. A
typical example is the requirement for chlorination of wastewaters before discharge
and of drinking waters prior to treatment. Chlorination of certain organic materials
produces the trihalomethanes, which are on the list of priority pollutants. If chlorination of wastewater is not practiced, there will be fewer trihalomethanes produced.
However, the downstream area will have a higher disease potential because the
pathogenic organisms have not been reduced sufficiently. Similarly, in the water
intake, where poor-quality water is used as the drinking water supply,
prechlorination has been practiced to reduce the number of pathogenic organisms.
This can result in an increase in the trihalomethane content. In order to reduce
trihalomethane production, it is recommended that prechlorination not be applied,
but that secondary chlorination, after the removal of the organics, be the only point
for chlorination. This of course will reduce the production of the trihalomethanes,
but may increase the potential for the carryover of pathogenic organisms in the water
supply system. Similarly, several heavy metals are on the list of priority pollutants.
Some of these metals have been shown to be essential growth factors in very low
concentrations. However, at only slightly higher concentrations, they have been
found to be detrimental. Thus, very close tolerance limits must be set, and it is
difficult to establish safe levels and still prevent the occurrence of unsafe levels.
Another problem comes in the determination of the priority pollutants. The
regulations call for the banning of any substance suspected of being carcinogenic
or mutagenic. This usually is based upon feeding massive doses to a susceptible
species of mice or rats. If any carcinogenic or mutagenic effects are found, that
substance is placed on the priority pollutant list. There is, however, no guarantee that
what is harmful to rats and mice will also be harmful to humans. Also it is difficult to
extrapolate to humans the concentration limits that were harmful to the animals.
A regulatory agency must of necessity be extremely conservative. Therefore, any
substance even suspected of being harmful must be included on this list. Environmental conditions are to be maintained so that even the most susceptible individual is
protected from any harm. Thus, the most conservative values and the lowest levels
are considered in listing the priority pollutants and their concentration limits.
One of the major problems in the evaluation of priority pollutants is the impact of
long-term use of any substance. There may be no immediate symptoms, but symptoms may occur after long periods of time, in another generation, or in a very indirect
manner. For example, the use of certain drugs (thalidomide) for increased fertility
has been shown to have an impact on the offspring of the second generation of
persons who used these drugs. Findings such as these take 20–25 yr to evaluate.
288
D. B. Aulenbach et al.