7.5: Measuring Risk

If I had to write on a post-it how environmental health workers assess risk, I'd jot down two phrases: the dose makes the poison and risk-benefit analysis.

The golden rule of toxicology: The dose makes the poison

Let's talk about "the dose makes the poison" first. This relates to hazard mapping. Just as something in the workplace that is hazardous in large quantities might not be hazardous in small quantities (for example, a match is less hazardous than a pizza oven), the dose of any kind of chemical or toxin makes all the difference between whether it is dangerous (poisonous, even) - or not.

One of the key tasks in toxicology (the study of toxins) is to determine how much of any given chemical can harm us. We are not able to live in a world without chemicals - even if we didn't produce gobs of them (for example, the chemicals in personal care products), we would still be exposed to naturally-occurring chemicals in the environment. The Environmental Protection Agency (EPA) tests about 1,500 new chemicals every year - yet upwards of 80,000 chemicals are used in commercial production in the US alone.

Many, many chemicals we come in contact with have not been tested, or not tested enough.

Even so, toxicologists do important work to determine what dose is safe - or in the case of medicines, even therapeutic. Anything can be a poison in too high a dose - even water. You may have heard of cases of people chugging gallons of water and having to be hospitalized as a result. The opposite is not true, however - it is not true that everything is safe in a small-enough dose (as there are some chemicals that are not safe in any dose).

The dose makes the poison. How much of something enters the body makes all the difference in how the body will respond.

Whose body? When measuring the safety of a chemical, researchers have to take into account that there are populations that are especially vulnerable - babies, children, elders, people with suppressed immune systems, etc. A calculation is made, for example, that if 10 mg is the safe dose for healthy adult, then a child should be exposed to no more than 10% of that.

Chemicals that were already in use before the 1970's wave of environmental regulations often have not been tested or assessed for safety. Industry was allowed to continue using what it had been using in the past, unless some new evidence emerged of its danger.

Tylenol (acetaminophen) is an important example to consider. Tylenol is very effective and very safe at the prescribed dose. It often causes less stomach pain than aspirin or ibuprofen, and it can be used in babies and children (aspirin, on the other hand, is not safe for children). It is frequently recommended by doctors and physicians, for these reasons. Yet Tylenol is responsible for a surprisingly high number of deaths every year in the US - something like 400 a year. This is because it is poisonous at higher-than-recommended doses. For someone who has a weakened liver already (due to illness or alcohol consumption), or for a child, taking as little as one or two extra pills of Tylenol beyond the recommended dose, can be poisonous. In general, we prefer to see medications where there is a bigger distance between an effective dose (say, 400 mg) and a dangerous dose (600 or 800 mg). But because Tylenol has been in use for a long time - and it does a good job, at the right dose - it continues to be widely prescribed and sold over the counter. (For more about the risks of Tylenol, you might want to listen to this optional episode of This American Life.)

Risk-Benefit Analysis

It is impossible to remove all hazards, all risks. The key question is, what risks are acceptable to us? And why?

Risk-benefit analysis is something we all do, informally. For example, if I go in late to work, there is a risk I will get in trouble. However there is a benefit to me too - maybe 15 extra minutes of sleep, or a chance to take the later train that isn't crowded. If the risk associated with arriving late is very great - say, I might get fired - then it is likely to outweigh the benefit. If the risk is small - I might get a nasty look from the boss - then the benefit starts to seem more attractive. If the benefit is very great - say, I go in late because I'm interviewing for a better job - then that could outweigh even a steep risk (like getting fired).

Environmental health professionals define and quantify the risks and benefits associated with different human activities and with different technological solutions. In addition to understanding science and public health and engineering, environmental health professionals may benefit from some studying math and economics, too.

Risk-benefit analysis also is reflected in discussions of environmental justice. We have to ask, whose risk? whose benefit? Communities are often presented with a choice about an environmental hazard that is phrased as jobs versus environment. That is, the benefit of industry is that it offers jobs, and the risk is that it pollutes. In many cases, this can be a false choice, when there is a third option available (even if it is more expensive), such as safe clean jobs in an industry that has minimized pollution to an acceptable level.

Figure \(\PageIndex{1}\): The 4 Step Risk Assessment Process. (Public Domain; EPA via https://www.epa.gov/risk/conducting-...isk-assessment)

Risk assessment typically involves four steps:

1. Hazard Identification: What health problems are caused by the pollutant in question?
2. Dose-Response Assessment: What are the health problems at different exposures? (at some doses, even a hazardous substance may be safe)
3. Exposure Assessment: How much of the pollutant are people exposed to during a specific time period? How many people are exposed?
4. Risk Characterization: What is the extra risk of health problems in the exposed population? (For example, while cancer may already be a risk, the exposure to asbestos increases the risk of cancer, so we want to quantify that "extra risk" associated with the asbestos exposure.)

Knowing how large the risk is, we can weigh it against the benefits that generating the pollution might bring. That decision is often a political decision, as there is no one scientific measure to determine if a certain benefit is worth the risk.

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