Have you ever wondered why some people get sick after being exposed to environmental hazards and others don't? For example, maybe you've known someone in your life who is a lifelong smoker and even after a life of smoking, seems relatively healthy. Whereas other people who smoked for a much shorter period died as a result of lung cancer. Well, part of the answer to this question can be provided by the concept of susceptibility, let's explore that. So, susceptibility is basically intrinsic individual or group level responses. I'll give you the EPA's definition of susceptibility here. It's differences in risk resulting from variation in both toxicity response, sensitivity and exposure as a result of sex, life, stage and behavior. Now, let's unpack that a little bit. Basically, we're talking about variations in toxicity response and exposure, leading to a situation like we have here. So, in this graph, the outcome here on the Y axis goes from no outcome to some adverse health outcome at the top of the Y axis. And on the X axis here, we have environmental conditions from negative. Meaning basically, we don't have an exposure, sort of the absence of an environmental hazard up to positive, meaning there's some environmental hazard present. Now, if we focus on someone who's not susceptible, their likelihood of getting a disease may be reasonably flat, regardless of what the actual exposure is to a particular environmental hazard. But the red line that you see sweeping up from the lower left corner to the upper right would show a susceptible individual. Where there's low exposure situation, they may have almost no risk of having a health impact. But as exposure goes up, some aspects or characteristics about them make them more and more or increasingly susceptible to that health outcome. Susceptibility is basically determined by two sets of factors, those are innate and acquired. So, innate our biological factors that cannot be changed with the exception of sex and acquired are exposure related factors that can, in most cases, be changed. Let's look at these in a little more detail. Let's dive into the innate factors a little bit more. So, these are factors or attributes about a person or population that can lead to a higher health risk at a given exposure level. And these factors include things like age and life stage, sex, race and ethnicity, genetic polymorphisms as well as toxicokinetic differences. And by that I mean individual factors or differences that can affect how easily different people absorb a chemical and metabolize it and excrete it. All of those processes are intimately tied to how toxic that chemical might be to those individuals. Now, if we talk about acquired factors, these include things like disease status, socioeconomic status, nutrition status and lifestyle. So with disease status, it's possible that if a person has a pre existing disease, that may make them more susceptible to some other health effect. And a classic example here would be an individual with asthma might be more susceptible to exposure to air pollutants. With regards to socioeconomic status, we know there is greater exposure, as well as a higher disease burden among low income communities and communities of color. For nutrition status, we know that diet can either build or improve your immune system, or it can harm your immune system. And finally, with regards to lifestyle, we know that things like exercise, smoking and alcohol and drug use can either build or harm your immune system. And that, in turn, is going to influence your susceptibility to disease. Now, if we think about certain populations that might be considered susceptible, I'll list four major ones here, so children are a great example of a susceptible population, as are the elderly. Minority groups are also highly susceptible to certain types of hazards. And as I mentioned, people or individuals with certain specific genetic traits may be uniquely susceptible as well. So, I bring these things up because in a public health context, we need to keep an eye out for these susceptible populations. When we're doing, for example an exposure assessment, we need to know what are the exposures, particularly among these susceptible populations, where we're most likely to see adverse health impacts. And where as a result, we need to target our intervention efforts first.
