Perhaps you're health but are told by your doctor, after a routine blood test, that you should take statins to combat high cholesterol. Fine. But then you start wondering what else you should be testing for in your blood. After all, you've been reading about all those toxic chemicals that invade our daily lives -- nasty-sounding things ranging from pesticides and PCBs to heavy metals and flame retardants.
Ah...those flame retardants. Polybrominated diphenyl ethers (PBDEs) have been protecting fabrics, furniture, computers, and various other consumer items from fire since the 1970s, so it comes as no great surprise that their levels in the environment and in humans have been steadily rising. Toxicological studies have shown that PBDEs can impair reflexes and learning abilities in rodents and can also delay puberty in the animals by interfering with thyroid function. Shouldn't you know if PBDEs are present in your blood? So you convince your doctor to find a lab that can carry out this analysis and off goes a sample. The results come back, and you find out you have 15 nanograms/L in your blood plasma. Or 15,000 picograms per liter. Wow! This may make you hot under the collar, but you think it means you won't burst into flames.
But what realistically does this value mean? Well, it certainly means that chemists have amazing analytical capabilities. Since you have about five liters of blood in your body, you now know that you are harboring 60 nanograms of flame retardant. That is 0.000000060 grams, or roughly 1/20,000th the mass of a grain of sand. Pretty impressive technology! But what does it say about any health risk? Without further information, not much.
The presence of a chemical in the blood does not equate to the presence of risk. As everyone hopefully understands, only the dose makes the poison. We certainly would be concerned if we had data suggesting that patients with some sort of disease were more likely to have higher blood levels of PBDEs than the rest of the population, and we would then want to know at what blood levels risk becomes significant. But we do not have such data. Not for PBDEs, nor for the numerous potential toxins that can now be measured in the blood, often down to the levels of parts per trillion. What is a part per trillion? Well, it's one second in 32,000 years!
We need to keep these concepts in mind when we are confronted with headlines like "Canadians' Blood Holds Chemical Cocktail." Environmental Defence, a Toronto-based organization, commissioned an analysis of blood taken from eleven Canadians, an analysis that generated a great deal of media interest. Some 88 substances were tested for, and on average half of these were detected in each volunteer. Environmental Defence's forty-two-page-long report, sensationally titled "Toxic Nation," provides a good discussion of the chemicals tested for, their possible sources, and amounts detected. But in the whole report there is not one single mention of any reference value for purposes of comparison and safety evaluation! Not even when these values are known. For example, the median value of mercury detected was well below 30 nmol/L, the level at which we should become concerned, according to the U.S. Centers for Disease Control (CDC).
And let's remember something else. The toxicity of a substance depends on its specific chemistry, not its source. Natural substances, such as acetaldehyde or estragole, are potent rodent carcinogens. Where can they be found? In apples! Of course nobody in their right mind would suggest that apples cause cancer, because obviously these compounds are present in trace amounts. But our sophisticated analytical techniques are capable of detecting them in people and could possibly give rise to scientifically accurate but meaningless headlines about how biomonitoring has revealed the presence of apple carcinogens in human blood.
All of this is not meant to suggest that biomonitoring, such as that carried out by Environmental Defence, or more meaningfully by the Centers for Disease Control, is not important. It is. Because it does provide information that may eventually be useful. That's why every two years, the CDC selects a couple of thousand people at random and carries out a comprehensive urine and blood analysis for some 150 different compounds that may enter the body from the environment. With such data, we may be able to determine whether these chemicals have any connection at all to conditions such as autism, asthma, testicular cancer, and low sperm count. Could phthalates leaching from plastics be involved? Or pyrethroid insecticides, residues of which are widespread? Or maybe even flame retardants? Perhaps we will one day find long-feared chemical culprits, but the mere presence of a substance in the blood, without any reference value, is not a cause for panic. Of course, neither should we try to sweep potential problems under the carpet. We should weigh risks and benefits -- keeping in mind, for instance, that PBDEs are estimated to save some 300 lives per year.
And in case you missed the news that came out the same week as "Toxic Nation," average life expectancy in Canada reached a record high in 2005. Hmmm...could it be because of all the chemicals in our blood?
Dr. Joe Schwarcz is Director of the McGill University Office for Science and Society.
