Mercury: An Ancient and Modern Public Health Threat
- Occupational Exposures
It has been known for centuries that over-exposure to mercury can cause severe neurological and neuropsychological effects. The signs and symptoms of mercury poisoning were described in mercury miners as early as the 1500s. The other significant occupational exposure to mercury was among the makers of felt hats. Mercury was used in that industry beginning in the 1600s. The famous reference to the Mad Hatter in Alice in Wonderland reflects the knowledge that mercury exposure can cause emotional and psychological effects such as irritability, apprehension, and restlessness.
When the effects of occupational mercury exposure became better known, steps were taken to limit mercury exposure in the workplace. It is now uncommon (but not impossible) for workers to be exposed to mercury in amounts great enough to cause damage to the central or peripheral nervous systems.
- Environmental Exposures
Two more recent public health disasters have highlighted the toxic effects of mercury exposure. In the 1950s and 1960s in Japan, large amounts of mercury were released from industrial sources into the waters of Minamata Bay. Mercury bioaccumulates in the ecosystem, so fish (a major food source of people living there) became contaminated with mercury. In this incident, 700 people died and 9,000 suffered severe neurological effects.
Mercury was formerly used as a fungicide to treat seed grains. In 1971, seeds treated with mercury were sent to Iraq to ease a food shortage there. Although the seeds were meant to be planted to raise grain, Iraqis either ate the seeds themselves or ground them to make flour for bread. Hundreds of people died and as many as 6,000 were affected. Neurological effects seen there were similar to those experienced in Japan.
- Current Concerns
Mercury exposure continues to be a public health concern in three main areas. One is the safety of consuming certain types of fish. Another is the risk of mercury exposure from dental amalgams used to fill cavities. A final area is the use of mercury-containing preservatives in vaccines.
Mercury in Fish
Mercury is ubiquitous in the environment. It can enter the environment from both natural and man-made sources. Mercury emitted into the air and discharged into waters can end up in the oceans where, just like in Minamata Bay, it can accumulate in the food chain. The fish most likely to contain elevated levels of mercury are those at or near the top of the aquatic food chain such as sharks, mackerel, and tuna. While eating fish is healthy in general, the Federal Drug Administration has advised that pregnant women should avoid eating those types of fish and that consumption of those fish should be limited in nursing mothers and young children.
Mercury in Dental Fillings
Dental amalgams can contain up to 50% mercury. Once used to fill a cavity in a tooth, this mercury is slowly released into the mouth from which it can be inhaled and be absorbed into the blood. Concern has been expressed about the effects of long-term exposure to mercury via this route. Most studies have shown the amounts of mercury released into the body in this manner do not have adverse effects, even for children who had pre-natal exposure from their mother’s dental amalgams.
Mercury in Vaccines
More controversial is the presence of a mercury-containing preservative called thimerosal in some vaccines given to small children. Some parents have become concerned that mercury in vaccines can cause adverse health effects, most notably autism, in their children. While scientific studies have shown that this is not the case, the media (traditional and social) still raise fears.
In some areas of the US, more that 5% of children go without being immunized against the common diseases of childhood. This is the real health threat. Immunization protects not only the individual children getting the vaccine, but also the entire community. Unvaccinated children and adults represent a reservoir for infections. This infection risk is particularly great for pregnant women and for children less than a year old who are too young to be vaccinated.
Other Present-Day Sources of Mercury
One final source of present-day exposure to mercury deserves comment. While mercury has been phased out of many products and industrial processes, some new uses have emerged. The most important of these is the development of compact fluorescent light (CFL) bulbs which have largely replaced the incandescent variety of these products. CLFs and other fluorescent light bulbs contain mercury and when broken or discarded into the municipal waste stream will release mercury into the personal and general environment. Mercury is also present in thermostat switches, old-style blood pressure measuring equipment, laboratory equipment, and heating and air conditioning systems.
- Summary and Conclusions
Mercury exposure can be unhealthy and even deadly. It is prudent to handle this metal with caution. It is also prudent to rely on scientific information and not what’s on Facebook or what your neighbor thinks when making health risk decisions. For example, avoiding vaccinations or not eating any fish are worse for personal and public health, not better.