Ecotoxicity and the enormity of tiny-ness
Dr Trevor Hancock
12 March 2019
Forty years ago Ross Hume Hall, then a professor of biochemistry at McMaster University, and Donald Chant, a University of Toronto professor of zoology and one of the founders of Pollution Probe, co-authored an important report on ecotoxicity for the Canadian Environmental Advisory Council. “The whole environment, including humans, is being contaminated in a sea of chemicals”, they wrote; “The term for this environmental defilement is ecotoxicity.”
One of the key concepts underlying ecotoxicity, they went on to say, is the “enormity of tiny-ness” in biological systems. First, they helped us understand just how tiny measures such as parts per million are: “One part per million (ppm) is equivalent to one inch in 16 miles, one minute in 2 years”.
In our daily lives, such small amounts are insignificant. If you are an engineer building a one-kilometre long bridge, I suspect a difference in length of one millimeter – a difference of one part per million – is not very important. And if you had a million dollars, I doubt you would worry about a difference of one cent, which is 10 parts per billion (ppb) – a ppb being one thousand times smaller than a ppm, so one inch in 16,000 miles, one minute in 2,000 years.
But in biology it’s different; such tiny amounts do matter. For example, our daily requirement for Vitamin B12 is about 2 – 3 micrograms per day, and average weight for adult humans globally is 62 kg. So we need less than 1 ten-billionth of our body weight of B12 daily – but without it we will develop a life-threatening disease, pernicious anaemia.
On the other hand, tiny amounts of chemicals can also be harmful. Take dioxins, for example, which are mainly “unwanted by-products of a wide range of manufacturing processes including smelting, chlorine bleaching of paper pulp and the manufacturing of some herbicides and pesticides”, according to the World Health Organization (WHO), although there are also natural sources. Dioxins are “highly toxic and can cause reproductive and developmental problems, damage the immune system, interfere with hormones and also cause cancer”, the WHO states.
In 2001 a joint expert committee of the WHO and the UN’s Food and Agriculture Organization established what’s called a provisional tolerable monthly intake of these chemicals (the amount we can ingest without detectable health effects) of 70 picograms/kg per month, which is about 120 picograms per day for a 60 kg adult. Since a picogram is one-millionth of a microgram, this level is about 20 thousand times less than the daily dose of Vitamin B12 needed to keep us healthy.
Dioxins are a family of chemicals that are part of the ‘dirty dozen’ – twelve persistent organic pollutants (POPs) that are so toxic that they are covered by the 2001 Stockholm Convention, a UN treaty intended to protect human health and the environment. The Convention describes POPs as “chemicals that remain intact in the environment for long periods, become widely distributed geographically, accumulate in the fatty tissue of humans and wildlife, and have harmful impacts on human health or on the environment”.
Here we have other important aspects of ecotoxicity. First, these chemicals are new to nature and cannot be easily broken down, so they persist. Second, they are taken up by and stored in fat. Third, because they persist and accumulate in fatty tissues, the seemingly tiny concentrations of these pollutants that we emit are re-concentrated by nature and presented back to us through the food chain. For dioxins, for example, WHO notes that “more than 90 percent of human exposure is through food, mainly meat and dairy products, fish and shellfish”.
This is because when a tiny fish eats plankton, it gets a dose of POPs that are then stored in its fats and oils. It is then eaten by a small fish, which is eaten by a bigger fish – and at each step, the amount and concentration of the POP increases. Then a seal eats the largest fish – and we eat the seal. A chart in the 2010 World Ocean Review shows the concentration of one POP – PCB – in seal fat is 80 million times the levels in sea water. This is another way in which tiny amounts can become enormous.
© Trevor Hancock, 2019