Environmental Engineering Reference
In-Depth Information
be considered. The crude death rate has decreased from 366.4 per 100,000 pop-
ulation in 1960 to 314.2 in 1988. The age-adjusted rate decreased from a peak
of 307.4 in 1950 to 134.6 in 1996, an overall decline of 56 percent. This decline
is due to a number of factors, including a decrease in the number of adults who
smoke cigarettes, better control of hypertension, less ingestion of cholesterol and
control of cholesterol levels, and improvements in medical care.
Methemoglobinemia
The presence of more than 45 mg/1 nitrates (10 mg/1 as N), the standard for
drinking water, appears to be the cause of methemoglobinemia, or “blue baby.”
The disease is largely confined to infants less than three months old but may
affect children up to age 6. It is caused by the bacterial conversion of the nitrate
ion ingested in water, formula, and other food to nitrite. Nitrite then converts
hemoglobin, the blood pigment that carries oxygen from the lungs to the tissues,
to methemoglobin. Because the altered pigment no longer can transport oxygen,
the physiologic effect is oxygen deprivation, or suffocation. Methemoglobinemia
is not a problem in adults, as the stomach pH is normally less than 4, whereas the
pH is generally higher in infants, allowing nitrate-reducing bacteria to survive.
Dental Caries
Fluoride deficiency is associated with dental caries and osteoporosis.
77
Water
containing 0.8 to 1.7 mg/1 natural or artificially added fluoride is beneficial to
children during the period they are developing permanent teeth. The incidence
of dental cavities, or tooth decay, is reduced by about 60 percent. The maxi-
mum fluoride concentration permissible in drinking water is 4.0 mg/1. Optimum
fluoride levels in drinking water for caries control, based on the annual average
of the maximum daily air temperature for the location of the community water
system, are as follows:
Temperature (
◦
F)
Fluoride Level (mg/1)
53.7 and below
1.2
53.8 - 58.3
1.1
58.4 - 63.8
1.0
63.9 - 70.6
0.9
70.7 - 79.2
0.8
79.3 - 90.5
0.7
An alternate to community water fluoridation is a 1-minute mouth rinse by
children once a week; it is reported to reduce tooth decay by about one-third or
more. The mouth rinse also appears to be beneficial to adults in the prevention
of dental caries. Other alternatives include fluoridation of school water supplies
if there is an onsite water supply, use of fluoride toothpaste, drops and tablets,
