LEAD IN AIR

LEAD IN AIR

Natural and anthropogenic sources of lead in the atmosphere and their dispersion have been discussed in Chapter 6. Direct inhalation of lead-bearing particles in air is one route of exposure for both humans and other animals. For most people, this is a minor exposure route, the majority of lead being taken in by ingestion (described later). However, leaded air particles fall to earth and contribute to lead levels in dusts, which can contaminate foods, hands, etc and thus be ingested. People exposed to lead fume at work can absorb significant amounts by inhalation if protective equipment is not worn and good working practices not followed.
The World Health Organisation set an air quality guideline for lead of between 0.5 and 1.0μg/m3 as an annual average (WHO, 1987), though a WHO working group recently recommended revising this guideline to 0.5μg/m3 (WHO, in press, cited in DETR, 1998). The EU lead in air standard is currently 0.5μg/m3. Ambient lead levels in air peaked in the early to mid 1980s in Britain. Annual average concentrations of airborne lead at the kerbside of a busy London road were around 1.4μg/m3, and in 1985 were still higher in Manchester (2.0μg/m3); general urban levels of lead were in the range of 0.2-1.0μg/m3. Since then, lead levels in urban areas have significantly declined, as the use of leaded petrol has been phased out. In the UK in 1995-6, the urban and kerbside lead levels measured were not above 0.2μg/m3, well within the WHO recommended levels. It is not expected that urban areas will now have lead levels above the lower WHO limit of 0.5μg/m3, though two industrial sites still exceeded this level. The upper WHO limit of 1.0μg/m3 was not exceeded at any of the 24 lead monitoring sites (chosen to represent urban, urban kerbside, rural and industrial locations) (UK DETR, 1998).
The relationship between lead levels in the air, and levels of lead absorbed by the body (blood lead levels) is complex, but evidence suggests that a standard for the general population should not be above 1μg/m3 (UK DETR, 1998). To allow for some individuals being more vulnerable, the UK Expert Panel on Air Quality Standards recommended a safety factor of 50% and a further safety factor of 50% to allow for uncertainties, resulting in a recommended value of 0.25μg/m3. It is believed at this level any effects on health, even to vulnerable individuals, will be too small to be detectable. This recommendation has, however, been criticised by industry as not being based on sound science.

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LEAD EXPOSURE IN HUMANS

LEAD EXPOSURE IN HUMANS
Levels of lead exposure in the general population have fallen in the USA and Western Europe, with average national values for blood lead in the EU in the 1990s mostly well below 10μg/dl. Decreased exposure from several sources has brought this about, though it is believed that the reduction of leaded petrol is a major contributory factor. Other measures have included: improved water treatments reducing plumbosolvency; phasing out of leaded solders in food cans; and phasing out of the use of leaded paints. Improved industrial practices have resulted in lower emissions. A small proportion of individuals continue to receive doses of lead which are deemed unhealthy, particularly in the developing World and in Eastern Europe. The most highly exposed individuals tend to be the more deprived members of society. Occupational exposure has also decreased dramatically in the western world with improved technology, hygiene and management practices.
EFFECTS OF LEAD EXPOSURE ON HUMAN HEALTH
A small number of adults occupationally exposed to lead have in the past shown increased risk of kidney damage, nerve damage, infertility and, possibly, a small increase in blood pressure and the risk of contracting certain cancers at high levels of exposure. However, today such effects are rarely observed. The greatest concern for the general population is that lower levels of lead exposure, which some of the general population may receive, appear to cause a small decrease in the intellectual development of young children. Children are more vulnerable because their nervous system is developing; they absorb more lead than adults because of behavioural and physiological differences. There is no accepted threshold level, but the body of evidence to date does not find any effect below 10μg/dl blood lead. Individuals whose diet is lacking in iron or calcium absorb more lead than those who are well nourished.
ECOTOXICITY
Lead can have adverse effects on living organisms. High doses can interfere with some biochemical processes required for normal functioning. Most lead compounds have low solubilities in water and are not readily absorbed by most living organisms. Soluble compounds of lead can readily be taken in, and have been studied most widely (particularly for aquatic organisms). However, some organisms (such as molluscs) can absorb solid lead compounds from sediments, and there is little data available on the toxicity of this. Tetraethyl lead is much more toxic than inorganic lead compounds, but it breaks down quickly in the environment. Some aquatic species bioaccumulate lead, but there is no evidence for biomagnification at higher levels in food chains. Lead in soil generally does not have great effects on plants or earthworms except at highly elevated concentrations; its potency varies with soil type. In general, the bioavailability and toxicity of lead compounds are greater in acidic conditions, and less in alkaline or saline conditions. The major impact of lead on wildlife, particularly waterfowl, results from the ingestion of lead shot (from ammunition or fishing weights). This can cause acute lead poisoning, sometimes fatal. For this reason, the use of lead shot for small fishing weights, and in some cases, in ammunition, is restricted in an increasing number of countries.

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Lead Exposure

Lead Exposure

Lead can reach the biosphere, including humans, by a number of routes. The main exposure routes to humans are:
Food – This is a major source of lead intake for the adult population. Produce can be ontaminated from airborne deposition and lead-rich soil, though this can be reduced with careful washing. Previously, leaded solders in cans and wine bottle capsules contributed to lead intake in the general population, but these have been phased out. In the home, the use of lead crystal or ceramic tableware glazed with leaded glazes can cause a small contribution, but such articles manufactured in the Western World are tested to ensure that leaching is acceptably low.
Water – Old lead pipes, which are still in place in many dwellings, can slowly dissolve in some soft and acidic waters. Improved water treatment to reduce plumbosolvency, reduces the lead content of water to acceptable levels in the majority of dwellings. However, the problem is not eliminated without replacement of lead pipes within the home, which incurs expense to the homeowner. A WHO guideline of 10μg/l for drinking water is complied with in most cases.
Air – Direct absorption by inhalation is a minor exposure route for most people, though it can be significant to individuals occupationally exposed. Airborne lead-containing particles fall to earth, most within a short distance of source, and add to lead contents of dusts, soils and food. A weak link between levels of lead in air and exposed populations has been established. The WHO recommended an air quality standard in 1987 of 0.5-1.0μg/m3. The EU lead in air standard is currently 0.5μg/m3 and the UK has set an air quality target of 0.25μg/m3. Air lead levels are falling in Western Europe, as leaded petrol is being phased out. Elevated levels can be found in some industrial areas.
Soil and dust – Ingestion of soil and house dust is a major pathway for the exposure of young children to lead, due to “hand-to-mouth” activity. The major source of lead in house dust in older properties is leaded paint (now phased out, but still in situ in many dwellings). This is the major source of lead exposure to children living in older dwellings in the USA. Other contributions to dust are from airborne emissions, particularly in countries where leaded petrol is still used. Lead contents in house dusts in urban areas of the UK are declining. Lead contents of soils cannot be expected to decline for many years, as lead has a low mobility in soil.
Lead exposure levels in the general population have markedly declined over the past 30 or so years and by the 1990s average national levels for blood lead in the EU were mostly well below 10μg/dl.
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