It's only recently that the wisdom of putting mercury amalgam fillings into teeth has been questioned, as more and more evidence about the harmful environmental and health effects of mercury have come to light.
These days, dentists have to use expensive filter processes to capture the escaping mercury vapour from mercury amalgam fillings. And they have to go to considerable lengths to dispose of old fillings (now categorised as toxic waste) because mercury is recognised as a serious environmental hazard.
But while mercury is categorised as a hazardous substance in the environment, it's still officially deemed to be safe when it's put into our mouths.
The New Zealand Dental Association and the Ministry of Health insist that mercury amalgam fillings are safe, durable and effective, because only small amounts of mercury vapour are released from fillings, and do not cause any real harm.
But it's hard to understand how these claims can be verified when the World Health Organisation says there is no safe level of exposure to mercury, and that exposure to even minute amounts of mercury poses a significant health risk.
The problem is that when people chew, grind or brush their teeth, amalgam fillings release small amounts of mercury vapour which is absorbed into the lungs and passes into the bloodstream, where it circulates around the body.
The human body cannot break mercury down into less harmful material, so the mercury accumulates in various tissues and organs, such as the kidneys, thyroid and the brain, where it can cause neurological, renal and other health effects.
Mercury can also cross the placenta into a developing fetus, and pass into breast milk. This is a real concern, as children and developing fetuses are especially vulnerable to the adverse health effects of mercury. That's why many countries advise against its use for children, pregnant women and people with kidney disease.
Some people are more susceptible to mercury toxicity than others, and there have been persistent concerns that mercury accumulating in the brain could be a factor in the upsurge in Alzheimers disease.
As more and more information about its potentially harmful effects has come to light, countries such as Norway, Sweden, Finland and Denmark have banned mercury amalgam fillings.
Other countries, however, including New Zealand, insist they are safe and allow their widespread, unrestricted use.
There are no guidelines about the number of mercury treatments people can safely have, for example, and no monitoring of the levels of mercury amalgam in our population either.
And in an apparent breach of the Health and Disability Commissioner's Code of Consumer Rights, dentists are not required to advise consumers about the potential risks of mercury fillings, or of alternative options, so patients can exercise informed choice. To the contrary, if dentists were to suggest that removing amalgam could alleviate a disease, they would be deemed to be acting unethically and could be censured.
Our laissez-faire approach to mercury amalgam could change, however, as a result of a new United Nations Convention to phase out the use of mercury globally, which was signed by more than 100 countries, including New Zealand, last year.
The treaty, which will eventually be legally binding, calls on governments to phase out all sources of mercury, including mercury amalgam.
As there are perfectly durable and effective composite fillings that dentists can use instead of mercury amalgam, surely it's time to question why we aren't taking a more precautionary approach - and at the very least stipulating that dentists should not use mercury amalgam fillings in children, pregnant women and people with kidney disease?
Surely, too, it's time to question why the government considers mercury safe when it's used as a filling in teeth, but as an environmental hazard anywhere else?
Sue Kedgley is a former Green MP and current Wellington regional councillor.
