Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
of 2 : 1 for the metro and 5 : 2 for cars.
Liu et al. (1994b) conducted a study of carbon monoxide exposure
among Taipei commuters (adults and students) in 1990. Roadside and
in-vehicle measurements were made at the same time that commuters'
personal exposure was assessed. Concentrations of carbon monoxide were
measured for three transportation modes (bus, private car and
motorcycle) and three times of day (morning rush hour, midday and
evening rush hour). The ratio of in-vehicle to ambient concentrations
of carbon monoxide was roughly 6 : 5, overall.
As part of their study of carbon monoxide exposure, Liu et al.
performed a survey of commuting patterns in Taipei, for students and
adults. Adults had a significantly longer average commuting time than
students (1.4 h versus 0.8 h). Students commuted typically by walking
(58%) or by riding on public buses (29%). Adults commuted to work by
motorcycle (28%), public bus (26%), or in private cars (26%).
Commuters using public buses had the longest commuting times (1.8 h
for adult workers, and 1.2 h for students).
WHO recommended guidelines for carbon monoxide are 30 mg/m 3 as a
1-h mean, 60 mg/m 3 for a 30-min mean, and 100 mg/m 3 as a 15-min
mean. These guidelines are designed to prevent carboxy-haemoglobin
levels in the bloodstream from surpassing 2.5-3.0% in the non-smoking
population, and to protect people who are prone to heart problems.
According to the 1992 UNEP report of air pollution in megacities of
the world, the 1-h WHO guideline is routinely exceeded by a factor of
2-3 times in several cities in Asia (Amman, Bangkok, Jakarta,
Peshawar, Shanghai) and Latin America (Mexico City, Santiago, Lima)
(UNEP/WHO, 1992). Considering the exposure studies conducted in Mexico
City and Taipei, the stationary monitors are an underestimate of the
population at risk of elevated carbon monoxide levels.
12.4 Exposures and biomarkers
12.4.1 Exposure to lead and cadmium
Dose-response relationships exist for lead toxicity in children
and adults, and demonstrate that subtle effects begin at levels as low
as 1 µg/dl of lead in blood. Severe toxicity is associated with
blood-lead levels of 70 µg/dl or higher in children, and 100 µg/dl or
higher in adults. Toxicity symptoms include poisoning of the central
nervous system, causing convulsions, coma, and deep, irreversible
mental retardation. Functional changes in the peripheral nervous
system and anaemia can also occur at levels below 40 µg/dl.
Particulate lead present in gasoline (from the octane enhancer
tetraethyl lead) and bromine (from the lead scavenger ethylene
dibromide) have traditionally been used as tracers for mobile sources.
The WHO recommended ambient air quality guideline for lead is
1 µg/m 3 , a level routinely exceeded in many large Asian cities today
where lead is still permitted in gasoline. This guideline value is
based on the assumption that 98% of the general population will be
maintained below a blood level of 20 µg/litre, which is considered the
maximum acceptable concentration in blood.
Jimenez & Velasquez (1989) conducted a study in Manila,
Philippines to measure blood lead concentrations in children. In a
sample of 544 children, the average blood lead level was 22.8 µg/dl,
with approximately 8% of the children having levels greater than
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