Environmental Engineering Reference
In-Depth Information
consumption, similar to what we observe for neurodevel-
opmental outcomes.
In addition to benefi cial nutrients in fi sh, observational
studies of the effects of methylmercury exposure through
fi sh consumption may also be confounded by sociodemo-
graphic differences between individuals who choose to eat
fi sh and those who do not. These differences may predict
neurodevelopmental
or
cardiovascular outcomes for rea-
sons that are independent of either the nutrient or the con-
taminant exposures that occur from eating fi sh.
It is interesting to note that in some heavily polluted
areas, such as those near mercury mines in inland China,
where fi sh is rarely eaten, other foods may be more impor-
tant sources of methylmercury exposure than fi sh. For
example, rice grown in contaminated waters was found
to accumulate very high levels of methylmercury (Zhang
et al., 2010). Since rice does not contain the benefi cial
nutrients to offset the harmful effects of methylmercury,
toxicity may be even greater from this food source.
such as diphtheria, pertussis, tetanus, and hepatitis
far outweigh any threats from exposure to thimerosal
(Clements and McIntyre, 2006).
At high doses, the toxicity of ethylmercury appears simi-
lar to that of methylmercury (Magos et al. 1985; Risher
et al., 2002; Clarkson and Magos, 2006). However, the ref-
erence dose derived by the US Environmental Protection
Agency (EPA) for methylmercury may not be generalizable
to ethylmercury. One reason is that ethylmercury clears
more quickly from blood, with a half-life of about 20 days
in adults, as compared with 50 days for methylmercury
(IPCS, 1990; Clarkson and Magos, 2006). A study of infants
exposed to ethylmercury through vaccination suggested an
even shorter half-life of 3.7 days (Pichichero et al., 2008).
The faster clearance, combined with the episodic nature of
vaccine-related exposure, is unlikely to result in accumu-
lation of mercury comparable to what might occur from
steady exposure to methylmercury from fi sh consumption.
At the time when the FDA was conducting its review and
risk assessment, there was growing public concern that
mercury in vaccines was causing an epidemic of autism
in children (Baker, 2008). A review of studies on this topic
published from 1966 to 2004 was conducted by Parker et al.
(2006). The four studies that found evidence for a harmful
association were published by the same authors (e.g., Geier
and Geier, 2003), used overlapping data sets, and contained
critical methodologic fl aws. Studies of higher quality did
not support the hypothetical association. More recent
st udies of neuropsychological outcomes in children corrob-
orate the absence of evidence for effects of early exposure
to thimerosal on the central nervous system (Thompson
et al., 2007; Price et al., 2010). Furthermore, autism rates
have not decreased since the removal of thimerosal (Stehr-
Green et al., 2003; Fombonne et al., 2006; Schechter and
Grether, 2008; Hertz-Picciotto and Delwiche, 2009).
Despite the reassuring scientifi c evidence surrounding thi-
merosal, large segments of the population remain wary of
vaccination (Baker, 2008).
Ethylmercury in Vaccines
The US Food and Drug administration (FDA) regulations
require that preservatives be present in multidose vaccine
vials to prevent bacterial and fungal contamination (Ball
et al., 2001). Multidose vials are commonly used because
they are less expensive and require less storage space than
single-use vials. Since the, 1930s, ethylmercury in the form
of thimerosal (sodium ethylmercury thiosalicylate) has been
used as a vaccine preservative (Committee on Infectious Dis-
eases and Committee on Environmental Health, 1999).
The FDA Modernization Act of 1997 mandated a reas-
sessment of the risks of mercury-containing food and
drugs. In its analysis, it determined that the ethylmer-
cury dose associated with the recommended immuniza-
tion schedule for the fi rst 6 months of life could exceed
the US EPA reference dose for methylmercury—a simi-
lar organomercurial compound (Ball et al., 2001). As a
result, in July 1999, the American Academy of Pediatrics
and the US Public Health Service recommended remov-
ing thimerosal from childhood vaccines (US Centers
for Disease Control and Prevention [CDC], 1999). They
reasoned that although there is no evidence that thi-
merosal poses more than a minute risk (primarily due
to local hypersensitivity reactions), mercury is a known
neurotoxicant that should be avoided if at all possible.
A primary consideration was the desire to preserve the
public's trust in vaccine safety. By 2001, vaccine manu-
facturers in the United States and Europe had eliminated
most multidose infant vaccines containing thimerosal
(specifi cally, the diphtheria-pertussis tetanus and hepa-
titis B vaccines) (US FDA, 2008b). However, thimerosal
is still used widely in infant vaccines in countries that
do not have the resources to provide their populations
with single-dose preparations. In many of these places,
the potentially deadly threats from childhood diseases
Elemental Mercury in Dental Amalgams
Debate surrounding the use of mercury amalgams for den-
tal restoration dates back to the 19th century, when the fi rst
national association of dentists—the American Association
of Dental Surgeons—came out strongly against it because of
mercury's known toxicity (Goldwater, 1972). A larger group
of dentists, who eventually became the American Dental
Association, argued in favor of the value of this low-cost
material, largely dismissing the notion of health risks.
Amalgam tooth fi llings contain about 50% elemental
mercury and are a source of persistent, low-level expo-
sure to mercury vapor (IPCS, 1991). Exposure is thought
to occur primarily through inhalation (ATSDR, 1999;
Berglund, 1992). About 80% of inhaled mercury vapor is
retained by the body, predominantly in the kidneys and to
a lesser degree in the central nervous system; however, most
