Risk In Perspective: Hazards Are Not All Created Equal

This series is a collaboration between neuroscientist Alison Bernstein and biologist Iida Ruishalme. Errors in risk perception are at the core of so many issues in science communication that we think this is a critical topic to explore in detail. This series is cross-posted on SciMoms and Thoughtscapism.

Alison and Iida would like to thank Anne Martin for her graphic design work in translating our abstract ideas into graphics. Anne is a designer, illustrator, and researcher currently finishing her PhD in Neurobiology and Anatomy at the University of Utah. You can see her work through her website at hungrybraindesign.com and follow her on Twitter @thehungrybrain. She also runs a blog teaching researchers how to visually communicate their science at vizsi.com.

All hazards are not equal

We have a tendency to selectively pay attention to certain hazards. We also tend to consider all hazards we pay attention to as equally risky and all risks as equally harmful. However, hazards can affect different numbers of people (depending on exposure level, genetics, and a variety of other factors) and have very different severity of harm (from temporary skin irritation to death). The simplified graph below provides a general framework for thinking about hazards. Hazards are binary; they either are or are not a hazard. To determine how risky a hazard is and what to do about it, we must consider how many people are affected and how severe the harm is.

Looking at the graph below, we can roughly divide hazards into four categories based on how many people are harmed and the severity of the harm. Notice that whether something is natural or synthetic does not impact which category it falls into.

Graph showing Number of People Affected on the x axis and Severity of Harm on the y axis.

  1. The upper right contains hazards that affect many people and that can have severe consequences: cigarette smoking (synthetic) and sun exposure (natural). Both can cause severe harm (cancer and death) in a relatively large number of people.
  2. The lower right quadrant includes hazards that affect many people but that have less severe outcomes. Rhinoviruses (natural), for example, cause the common cold in a large proportion of the population each year. While you may feel terrible for a few days, this is, for the most part, a temporary and usually not very severe harm. Another example in this category is the chemicals used in pool water (synthetic). These can cause temporary eye or skin irritation in some people and everyone who swims in a pool is exposed.
  3. The upper left quadrant includes hazards that affect a few people, but with severe consequences. Allergens (natural) are an example of these hazards. A small proportion of the population is harmed by any specific allergen, but exposure can be a matter of life or death. Also included here are occupational exposures to industrial chemicals (synthetic). Very high occupational exposures in a variety of professions can cause severe health problems.
  4. The lower left quadrant represents hazards that only a few people are exposed to that also have less severe outcomes, such as hot chili peppers. Very few people choose to eat these peppers, but this mostly causes short-lived discomfort with no real long-term consequences (unless you eat one of the newest super hot peppers that can actually interfere with breathing or burn a hole in your esophagus).

Reducing the impact of hazards

If we use this framework, we can think about reducing the impact of hazards as trying to push hazards to the left and/or down in these quadrants. Measures to reduce the impact of hazards can aim to reduce the number of people exposed, the level of exposure in each person, or the harm caused by exposure.

For example, public health measures have decreased the number of people exposed to cigarette smoke by making it harder for people to smoke. Regulations have also reduced exposures and the severity of harm by removing or reducing some of the most harmful chemicals in cigarettes and adding filters.

For allergens, laws requiring the labeling of products containing allergens allow those with allergies to avoid allergens, reducing the number of people exposed. Efforts to generate low allergen versions of these foods are a way to potentially reduce the severity of harm. For occupational exposures, regulations have been implemented to ensure that the most toxic chemicals are no longer used or that proper precautions are taken in the workplace.

From a public health perspective, it makes sense to explore and understand a wide range of hazards. Then we can prioritize which risk factors to focus on, based on data about how many people are affected, the severity of harm, and other factors. The greater the harm and the more people a hazard affects, the more important it is for public health agencies to devote resources to find ways of reducing those risks. It is important to remember that some risks may be very hard to reduce or eliminate and that taking steps to do so may also introduce new risks that are as great, or greater. In some cases, increasing protective factors may have an even larger effect than on reducing risk factors. All of these measures have the effect of reducing the probability of harm (risk) from hazards.

Hazardous mistakes: Vaccine denial and alternative vaccine schedules

The rise of both vaccine denial and the use of alternative schedules highlights our tendency to view all hazards as equal. These choices are partly a result of considering the hazards of vaccines and the hazards of disease as equivalent in both severity and risk of harm.

Text infographic titled "Which is riskier, Pertussis or DTaP?" The most severe risk of Pertussis (death) is 10,000 times riskier than the most severe risk of the vaccine (allergic reaction).
The Mad Virologist has a great collection of infographics on risks of diseases and their vaccinations side by side.

Vaccine-preventable diseases belong in the top right quadrant of the hazard classification graphic. Vaccines move the hazard of these diseases to the upper left quadrant by drastically reducing the number of people affected by these diseases. In some cases, vaccines can also move this hazard down by reducing the severity of disease if the vaccine is not completely effective at preventing disease.

On the other hand, vaccines themselves have a generally low severity of harm in a small proportion of the population, putting them in the bottom left quadrant. These hazards are not equal in their likelihood of causing harm (risk) or in the severity of harm. However, in many parents’ minds, the extremely rare risks (real or imagined) associated with vaccines have become equal to or greater than of the very real risks of contracting vaccine-preventable diseases.

In addition, parents who choose to use a delayed or alternative vaccine schedule ignore the risk of increased time without immunity, the risk of following a schedule that has not undergone the same rigorous vetting as the CDC recommended vaccine schedule, and risks of exposure to illnesses caused by making more visits to the doctors’ office. There are specific examples where side effects from vaccinations are higher in older children, as explained in detail in this 2014 article by Tara Haelle in Scientific American, Delaying Vaccines Increases Risks—with No Added Benefits:

The risk of a febrile seizure following the MMR [vaccine] is approximately one case in 3,000 doses for children aged 12 to 15 months but one case in 1,500 doses for children aged 16 to 23 months “This study adds to the evidence that the best way to prevent disease and minimize side effects from vaccines is to vaccinate on the recommended schedule,” says Simon Hambidge, lead author of the study and the director of general pediatrics at Denver Health. Otherwise, he says, an undervaccinated child is left at risk of infectious disease for a longer period.

This article, Why it’s a bad idea to space out your child’s vaccination shots, addresses many concerns that parents have about the recommended schedule. At the core of vaccine hesitancy is a failure to recognize that the hazards of vaccines and the diseases they prevent do not pose the same levels of risk.

If you would like to read more about different aspects of risk perception, please see the other parts of the series, which this article belongs to:

Risk in Perspective

  1. The difference between hazard and risk is a critical distinction.
  2. All hazards are not equal.
  3. Zero risk and zero exposure are impossible expectations.
  4. Population risk is not the same as individual risk.

Header image of door with hazard sign by David Stillman via Flickr.