Page 132 - Fundamentals of Air Pollution

102 7. Effects on Human Health and Welfare

A. Lead and the Human Body
The major sources of airborne lead are leaded gasoline, incineration of
solid wastes and discarded oil, and certain manufacturing processes (6).
The populations most sensitive to lead exposure are young children and
fetuses. Lead can degrade renal function, impair hemoglobin synthesis,
and alter the nervous system. The neurobehavioral impairment of children's
intellectual development is a major concern for lead exposure. There are
two routes for the entry of lead—inhalation and ingestion. The importance
of each depends on the circumstances. As noted in Chapter 20, the U.S.
national ambient air quality standard for lead is based on the ingestion
route, which accounts for 80% of the allowed body burden, with only the
remaining 20% permissible via inhalation. Inhalation results in primary
exposure to airborne lead, whereas ingestion may result in secondary expo-
sure via contamination of the ingested material by atmospheric lead. When
lead is inhaled, some of it is absorbed directly into the bloodstream and a
fraction into the gastrointestinal tract through lung clearance mechanisms
that result in swallowing of mucus.
The absorption, distribution, and accumulation of lead in the human
body may be represented by a three-part model (6). The first part consists
of red blood cells, which move the lead to the other two parts, soft tissue
and bone. The blood cells and soft tissue, represented by the liver and
kidney, constitute the mobile part of the lead body burden, which can
fluctuate depending on the length of exposure to the pollutant. Lead accu-
mulation over a long period of time occurs in the bones, which store up
to 95% of the total body burden. However, the lead in soft tissue represents
a potentially greater toxicological hazard and is the more important compo-
nent of the lead body burden. Lead measured in the urine has been found
to be a good index of the amount of mobile lead in the body. The majority
of lead is eliminated from the body in the urine and feces, with smaller
amounts removed by sweat, hair, and nails.

B. Carbon Monoxide and the Human Body
The second example of an air pollutant that affects the total body burden
is carbon monoxide (CO). In addition to CO in ambient air, there are other
sources for inhalation. People who smoke have an elevated CO body burden
compared to nonsmokers. Individuals indoors may be exposed to elevated
levels of CO from incomplete combustion in heating or cooking stoves.
CO gas enters the human body by inhalation and is absorbed directly into
the bloodstream; the total body burden resides in the circulatory system.
The human body also produces CO by breakdown of hemoglobin. Hemo-
globin breakdown gives every individual a baseline level of CO in the
circulatory system. As the result of these factors, the body burden can
fluctuate over a time scale of hours.