What’s environmental toxicology?

It is the study of the harmful effects of substances on humans and other organisms in the environment.

To understand this properly, let us ponder over the following.

What’s a harmful effect? Anything that causes an organism to die, stops or delays their development or growth, changes their behavior… you get it. It can make organisms not function properly or reduce their ability to deal with stresses like extreme temperatures, less food, infections, etc.

What is a toxic substance? Pretty much everything and anything! Because, as Paracelsus said, the dose (quantity) makes the poison. Almost everything is poisonous and almost everything is safe (one of the exceptions is lead – there is believed to be no safe dose).

The most toxic substance on earth, the bacteria-produced botulinum toxin, can kill humans at a dose of 0.0000001 grams (or 0.0000000035 ounces), if injected. However, it is used in Botox. On the other hand, it takes 6 kilograms/6 liters (or 13 pounds/1.6 gallons) of water (consumed all at once) to kill a person.

https://upload.wikimedia.org/wikipedia/commons/f/fc/Paracelsus_1.jpg
Figure 1: Paracelsus is considered the Father of Toxicology {{PD-old}}

How do we know which substances are very toxic and which aren’t? Scientists look at how much of the substance is needed to cause a harmful effect. In the above example, very little of botulinum toxin can kill a human. So it is classified as a highly toxic substance. Since it takes a lot of water to kill a human, it is classified as a very lowly toxic substance. But what amount does it take to decide this?

Let’s do an experiment, as scientists often do. Let’s feed a substance to a population of guinea pigs (each weighing 1 kg or 2.2 lbs) until half of them die. The substance’s toxicity can be classified based on the amount it took to kill half the guinea pigs.

Highly toxic Moderately toxic Lowly toxic Very lowly toxic
0-50 mg

(0-0.002 oz)

50-500 mg

(0.002-0.02 oz)

500-5000 mg

(0.02-0.2 oz)

More than 5000 mg (more than 0.2 oz)

Since humans weigh around 70 times more than guinea pigs, we can roughly multiply all the numbers by 70 (notice change in units).

Highly toxic Moderately toxic Lowly toxic Very lowly toxic
0-3.5 g

(0-0.008 lb)

3.5-35 g

(0.008-0.08 lb)

35-350 g

(0.08-0.8 lb)

More than 350 g (more than 0.8 lb)

Body weights matter in toxicology. This is why medicine prescriptions often state that children should take a smaller dose.

Does a larger dose always cause more harm than a smaller dose? Mostly, yes. Larger the dose = larger the harmful effect. This is called a dose-response relationship (see Figure 2). But this doesn’t hold true for chemicals that are essential for survival. Low doses of vitamins (or oxygen, water, etc.) are bad for health, and so are high doses. There is an optimal range of vitamin (/essential chemical) doses that are safe — too low a dose causes deficiency and death and too high a dose causes toxicity and death (see Figure 3). Yikes!

Other exceptions to the “high dose high effect” relationship are some chemicals that disrupt the hormonal system. They carry out their activity at low doses but not at high doses. They are said to have an inverse dose-response relationship for that particular effect.

DRC
Figure 2: A standard dose-response relation. With increase in dose of chemical, there is an increase in response
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Figure 3: A non-standard dose-response relation. The very low and very high doses cause a harmful response

Is dose the only thing that decides the poison? No, other things like age, sex, timing, duration, and route of exposure, species, and target tissue also matter. Usually, the unborn, young, and the elderly are the most susceptible. Chemicals which mimic specific sex hormones can disproportionately affect one sex over the other. Longer exposure to chemicals can increase risk of certain diseases, different species have different breakdown enzymes that can alter their susceptibility, and so on. Other factors like genetics, health, diet (nutrition), exposure to microbes or other chemicals, also play critical roles in determining the toxicity of a chemical.

Importantly, the exposure dose (or concentration) also matters. A substance can be highly toxic (see tables above), but you won’t be harmed if you don’t encounter it or if you only encounter it as doses that are safe.

Are man-made substances more toxic? We don’t know the answer to that but the five most toxic substances on earth (according to one source) are naturally produced.

Who is most likely to be harmed? Surprisingly (or unsurprisingly if you have thought it through), humans are probably one of the most vulnerable species. As we are rarely restricted by geography or season, we can be exposed to many substances. But we can also take measures to avoid exposure. Species with low population numbers or that are ineffective at breaking down and excreting chemicals are probably worse off.

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