This is the first of a series of 3 posts in which I’ll try to explain the basic issues faced by scientists attempting to understand the fate of chemicals released into the environment. It’s particularly relevant to me because I’m currently grappling with what seem like fairly simple problems in my research, and a sense of perspective would be nice!
So, if you’ve ever wondered why there have been environmental disasters involving ‘safe’ chemicals, or why we might see adverse effects on living things in unanticipated ways, then this post might help you to understand the difficulties we face when we try to work out whether something is toxic, or whether it is benign. Considering that thousands of new chemical compounds are synthesised and released into the environment each year, we need a way to assess them to work out which ones could be the biggest problem.
A little disclaimer; while I am currently working in the field of toxicology, I don’t have a lengthy background in the field, so I’m summarising my personal understanding here. If anyone would like to dispute any points I make, or if I’ve missed something critical, comment away!
As I mentioned, I’m splitting these posts into three more readable chunks:
Part 1 will focus on the way chemicals affect different organisms.
Part 2 will focus on mixtures, synergism and antagonism in chemical cocktails.
Part 3 will focus on the environmental fate of contaminants, and the bigger picture.
So, what do I mean when I say ‘toxicant’? Any chemical can be toxic; whether it is or not will depend on the concentration and situation. For example, table salt is generally harmless to people unless we go silly with the potato crisps on a hot day and get dehydrated. However, a freshwater fish would suffer and die if we put it in a tank of salty water. Some chemicals, such as heavy metals, are more harmful in general than others, but it is important to remember that any chemical can have a toxic effect on a living organism in the right circumstances.
The current state of toxicity testing has two broad areas: those observing changes in molecular indicators, such as mutations in DNA, and those concerned with the overall effects of a toxicant on an organism. I’ll be focusing on the latter, because that’s what I’m the most familiar with. Most toxicity testing on living organisms has been performed on a small suite of ‘model’ organisms, including mice, water fleas (Daphnia), zebra fish, algae and bacteria. Testing ‘higher’ organisms, such as humans, is rarely possible, and there is a general shift away from using vertebrates as test subjects for ethical reasons.
However, in general, the effect of a chemical on one of these common test species is used as an indication of whether it will affect the entire group represented by the test species: zebra fish have a lot of responsibility on their little scaly shoulders!
Given the huge number of chemicals out there to be characterised, using the same small range of organisms from different groups is an appealing prospect, as we can use the results to classify chemicals in terms of how dangerous they are to vertebrates, insects or bacteria. An organo-chlorine which is capable of quickly killing insects but doesn’t appear harm a rat might be a good prospect for keeping pests from decimating crops. Right?
Well, we have a problem: different living organisms will respond differently to the same chemical. I won’t go into too much detail about the way chemicals exert toxic effects, but suffice to say there are almost as many ways to hurt a living cell as there are chemicals! Metals might bind to enzymes performing important functions, pesticides can stop cells from being able to respire, and detergents can break down cell membranes, letting the contents spill out in a microscopic explosion of bacteri-gore. These differences can be more pronounced across broad groups, such as vertebrate vs. invertebrate, or land vs. aquatic, but in general, there is always a level of uncertainty as to whether one species or even one individual will respond to a toxicant in the same way as another.
There are no easy solutions to this problem. The more information we gather about the way specific chemicals interact with living systems, the better we can predict any adverse effects they might have. Screening toxicants against a range of model organisms gives us a lot of information about their potential for negative effects on their associated group. A combination of data about the way a chemical works and its observed effects on a range of living things gives us a strong basis for classifying it and controlling the way it can be used.
In summary, for even a single chemical, ‘toxic’ is entirely based on the context: a cup of coffee is nice in the morning, but a shot of pure caffiene into the bloodstream would be disastrous, and your pet sea monkeys might not appreciate a warm Espresso in their tank. Using a range of organisms as test subjects and studying the way the chemical interacts with living things can give us a good idea of how it might affect a particular living thing, but it is impossible to get hard data for every chemical and every situation.
That’s problem number one: we’re all different! In the next post I’ll explain how the waters get muddied when chemicals start to mix – we all know a cup of wine, beer and spirits will make you sicker than a cup of just one of those things, and similar effects are found everywhere in the chemical world!