PRINCIPLES OF TOXICOLOGY - Biology East Borneo

14.5 SOURCES OF METAL EXPOSURE 335Humans often may be exposed to hazardous levels of metals in the environment through thealteration of natural biogeochemical cycles. For instance, the use of lead as a gasoline additive increasedthe amount of lead to which people in urban areas were exposed to levels far above what would normallybe inhaled if unleaded gasoline was used. The discontinuance of lead as an additive has dramaticallydecreased the impact of this exposure route, as well as the presence of lead in soils. However, thecontinued presence of lead-based paint and lead pipes (or lead plumbing solder) for transportingdrinking water has maintained lead exposure at far above background levels. The presence of lead inthe paint of old buildings, where it may be inhaled as paint dust or ingested by children swallowingpaint chips, continues to be a major public health problem despite active efforts in the public healthfield at local, state, and federal levels.The organomanganese compound, methylcyclopentadienyl manganese tricarbonyl (MMT), wasused as an antiknock additive to replace lead in gasoline, but serious concerns regarding its healtheffects have been raised as well. MMT was banned for this use in the United States in 1977. A courtdecision in 1995 ordered EPA to lift the ban and allowed the registration of MMT. Testing for healtheffects of this manganese compound is ongoing.People living near waste sites, mining operations, or smelters may be exposed to higher-than-backgroundlevels of metals in air, drinking water, and soil. Several incidents of massive public poisoningdue to accidental or merely ignorant environmental release of metal wastes have underscored thepotential dangers. Perhaps the most famous is the Minamata disease, named after the area of Japanwhere many cases of severe neurological impairment and death appeared among the population in the1950s. It was eventually discovered that mercury wastes discharged into the nearby bay from achloralkali plant were being bioaccumulated by the fish and shellfish. Since mercury is easily convertedinto the methyl form under common environmental conditions, and because the form persists for longperiods in biological tissues, a magnification of tissue mercury concentration up the aquatic food chainwould be expected. Thus, by the time the seafood was harvested for human consumption, it containedextremely toxic levels of methyl mercury, which were reflected in severe effects on adults and childrenin the area.Aside from such catastrophic epidemics of metal poisoning among the general public, exposure tohigh levels of metals is usually of greatest concern for workers in industries where metals are commonlyused. These include mining, processing and smelting, manufacturing, and waste disposal operations.Occupational exposure to some metals may be confined to specific industries. For instance, agriculturalworkers may have exposure to arsenic and mercury, which are ingredients in some herbicides andfungicides. Sheetmetal workers may be exposed to copper and aluminum dust particles. Gold, silver,platinum, and nickel are metals commonly handled by workers involved in manufacturing jewelry, andmay exert effects under some conditions of exposure.Although workers are often exposed to higher metal concentrations than members of the generalpublic, this exposure can be maintained at a safe level through proper enforcement of regulationsregarding exposure limits and workplace safety. Because workplace exposure is often confined to aspecific site with a specific population at risk, routine monitoring of exposure can be performed, withcorrective action taken as necessary to maintain safe limits.Indicators of Exposure (Biomarkers)In addition to overt signs of toxicity, exposure to metals may be verified and often quantified by specificbiomarkers of exposure. A biomarker of exposure is any measurable biological parameter that indicatesexposure to a toxic substance, whether it is an induced protein, enzyme, metabolite, or even the toxicsubstance itself. In assessing suspected metal exposure, the first step is usually to take blood and/orurine samples for analysis. Since metals cannot be metabolized beyond recognition, and many metalsare not normal constituents of biological samples, their detection in the samples discussed above certaindefined levels is a reliable indicator of recent metal exposure, and perhaps intoxication. Due to therelatively short half-life of most metals in the blood and urine, sampling is usually required within