Mercury is one of the most toxic substances around, yet it has been used for 60 years as a preservative - called thimerosal - in vaccinations we give to our children.
When challenged, health authorities around the world have always pulled out the usual rejoinder, presumably taught at medical school, 'there is no conclusive evidence that points to a link between thimerosal and ill health'.
Now that all rather depends on who's looking, and how they're looking, as these factors have a big influence on what it is that's found.
It's an issue that has been exercising the minds of the American health authorities in recent years (while their UK counterparts slumber on, so nothing new there, then). A study last year repeated the usual mantra of there being no causative link, a finding that has been challenged by no less an authority than the Food and Drug Administration, the US's drugs regulator.
To be accurate, the challenge comes from Eric Colman, one of the FDA's leading officials speaking in an individual capacity, but it remains a major challenge nonetheless.
In effect, Mr Colman says there has never been a thorough, systematic study that has proven the safety of thimerosal (and, surely, that has to be where the burden of proof should lie). Furthermore, last year's researchers took blood samples only seven days after the vaccine had been administered. Had they done so within three days, they would have found a mercury concentration in the infants' blood that was far beyond recognized safety levels.
In the UK, thimerosal (thiomersal) is still present in the triple DTP vaccination. The Department of Health has taken an interesting line on the whole business: because the MMR jab doesn't contain thimerosal, and it's the vaccine linked to autism, the DTP jab must be safe, they argue. Eh?
(Source: Lancet, 2003; 361: 698