Hazard vs Risk: EWG, Prop 65, and IARC Explained

What's the Difference Between Hazard and Risk?

When we want to know if something is toxic or non-toxic, we're really asking about risk. Right? We want to know if something is risky, or not risky, to our health.

The problem is, risk gets confused with 5 other things constantly. Every argument about whether something is toxic or not comes down to someone confusing one of these five other things for risk.

Those things are: hazard, exposure, dose, dose-response, and susceptibility. They are each an important part of the pathway that leads to risk, but none of them are risk on their own.

Here is how they are related visually: we'll dive into each one in more detail below and throughout the course.

The 6-Step Toxicology Framework, at a Glance

What Does Hazard Mean?

Hazard is the intrinsic capacity of a substance to cause harm under some conditions. It is a property of the substance or stressor itself, independent of whether you ever come into contact with it, how much you are exposed to, how susceptible you are, and how your body handles it. For example, arsenic sitting in a sealed bottle on a shelf in another country is high-hazard but zero-risk to you.

Are EWG Ratings Accurate?

If you're not already familiar, the Environmental Working Group (EWG) has databases rating thousands of personal care, sunscreen, and cleaning products for safety. But instead of going through all six steps of the above framework to arrive at a risk score for each ingredient (considering exposure, dose-response curves, and susceptibility) their ratings are based entirely on the first step: hazard.

The EWG lists cause a lot of controversy because most people don't know this, and don't know the difference between risk and hazard in the first place. Some doctors and scientists say the lists make people unnecessarily worried, because cleaning products that have high hazard scores look really risky, even when they aren't.

What Does an IARC "Group 2B Carcinogen" Classification Actually Mean?

The International Agency for Research on Cancer's (IARC) "Group 2B Carcinogen" is a hazard classification, not proof of human carcinogenicity risk. Aloe vera is in Group 2B, meaning it's a "possibly carcinogenic" hazard. This is because ingesting the yellow latex sap may cause cancer in rats. It does not mean that this has been shown to cause cancer in humans, and most people aren't exposed to the yellow sap part of aloe vera because they don't drink it. The clear gel portion of aloe vera is generally considered safe to drink and apply to your skin after sunburn. Group 2B status for aloe vera just means that the yellow gel portion of it warrants further study to find out if it drinking it could cause cancer in people.

Context matters a lot. None of this means EWG or IARC are bad. Hazard identification is an important first step. EWG started flagging PFAS as hazardous in products in 2001, a solid 20 years before the rest of the country showed interest. And they acknowledge that their ratings "do not account for the level of exposure or individual susceptibility, factors that determine actual health risks." IARC says the same.

For a hazard to potentially become a risk, you need to first be exposed to it, and we cover the Exposure step next.

What Does Exposure Mean?

Exposure is the event of contact between you and a substance. Without exposure, even the most hazardous substance poses no risk. Exposure has four important dimensions: route (ingestion, inhalation, dermal absorption), frequency (once vs. daily), duration (acute vs. chronic), and timing (during fetal development vs. adulthood). You cannot change a substance's hazard, but you can often change your exposure to it.

What Does a Prop 65 Warning Actually Mean?

A Prop 65 warning tells you a listed chemical simply exists somewhere in a product's chain. It does not tell you whether that chemical can leave the product, reach your body, by what route, at what concentration, or whether any meaningful contact is occurring at all.

The warning requirement covers someone's potential exposure to a carcinogen or reproductive toxicant. It covers not just you, the end user, but anyone involved in making the product. This is why solid wood furniture can carry a Prop 65 warning for wood dust: it's a hazard to the workers breathing in wood dust particles, not necessarily to you reading in your living room.

Companies applying these warnings are accounting for worst-case exposure scenarios. They are legally protecting themselves, asking, what if someone misuses the product in an extreme way and we get sued? What if someone tests this product and finds lead contamination on it from some other source? Neither Prop 65, nor the companies abiding by it, are describing your personal risk.

Prop 65 warnings confuse just about everyone, and even the creating agency has acknowledged they're difficult to interpret. You can read more in my Prop 65 Guide.

What is Dose?

Dose is how much of something is in your body. It says nothing about whether it's safe, harmful, or what health response might follow. It doesn't state your risk. Dose just says: it's there, and this is how much there is.

What Do "Sunscreen Chemicals Found in Blood" Headlines Actually Show?

The FDA published 2 studies between 2019 and 2020 showing that chemical sunscreen ingredients enter the bloodstream after a single day of use, and some of those chemicals stay for at least 21 days. The blood concentrations were above 0.5ng/mL, which is the level at which the FDA requires more studies need to be done to thoroughly assess safety. Importantly, the study did not look into whether or not these doses were harmful, just that the doses existed in the body. They concluded only that they deserve more investigation.

Social media treats this two ways. One way is the confirmation that chemical sunscreen harms people. The other way is the dismissive statement, "the dose is so tiny, you're being ridiculous." But both are wrong, because neither takes dose-response into account, as you'll see next.

What is Dose-Response?

It is our body's response to a dose of something. Detecting a chemical in blood doesn't automatically mean harm. We are porous beings, and a lot of the world passes through our bloodstreams. Sometimes our bodies know exactly how to clear that out, and it doesn't cause a problem. On the other hand, sometimes our body doesn't protect itself well, and health consequences follow. Most interestingly, our body's response doesn't always neatly follow the idea that a small dose is safe and a big dose is harmful.

There are three main ways the body responds to doses: the threshold model, the linear no-threshold (LNT) model, and the non-monotonic dose response (NMDR).

1. Threshold model

The threshold model means that below a certain level, the body has pathways designed and ready to clear a substance without harm. Oral vitamin D supplements are a perfect example. Your body easily manages normal doses of vitamin D, regulating storage to keep levels in a healthy range. But push past a certain limit (the threshold) with long-term high-dose supplementation, and your body gets overwhelmed. Toxicity follows.

2. Linear no-threshold (LNT) model

The model usually applies to carcinogens, where no amount is totally safe. The clearest example is ionizing radiation like X-rays and gamma rays. There is no dose low enough to produce zero cancer risk, and as cumulative exposure rises, so does that risk.

3. Non-monotonic dose-response (NMDR)

Non-monotonic dose-response (NMDR) curves have a U-shape because low doses can produce a high response, and higher doses can produce less of response. It's mind-boggling! This has been documented for a growing class of chemicals, particularly endocrine disruptors. The Endocrine Society's 2025 position statement notes that this pattern is now well enough established that it has significant consequences for how we do risk assessment. This means it's no longer considered a fringe position, as it once was.

Which brings us back to the sunscreen studies we started talking about in the Dose section. After the 2020 FDA paper about sunscreen dose in the blood came out, a handful of studies followed, looking at the dose-response of these sunscreen chemicals. They said, ok, at this level of oxybenzone in the blood, what happens?

What they found in a 2023 review that looked at 254 studies on oxybenzone was interesting. After someone applies sunscreen to their whole body once, the amount of oxybenzone that shows up in their blood can reach the same levels that disrupt hormones (specifically, estrogen and testosterone activity) in lab and animal studies. Whether that actually causes hormone problems in people is still an open question. Frustratingly, three years on, we still don't have much human data to answer it. But, dismissing these oxybenzone doses as safe is not the right answer either. With this U-shaped response possible, we just do not know yet.

What Is Susceptibility?

What's toxic for one person isn't necessarily toxic for all people. A chemical that's a modest concern for a healthy adult can be a much bigger deal for a pregnant woman or a baby.

Flame retardants in furniture foam are a good example. Studies have found toddler exposure levels nearly five times higher than their mothers', partly because young children spend more time on the floor and put their hands and toys in their mouths, which puts them in close contact with the dust that carries these chemicals. The timing matters as much as the dose: their brains and thyroids are actively forming, and exposure during this window has been linked to neurodevelopmental effects.

The complication of susceptibility is one of the main reasons EWG, IARC, and Prop 65 can't produce true risk assessments. Accounting for life stage, genetics, pre-existing conditions, sex, and body size makes risk assessment hard to do at scale. The framework you're learning lets you do it for yourself.

What Is Risk?

Risk is the final output of all five previous steps: If you're exposed to a hazard and experience a high enough dose of it in your body, its dose-response curve means harm at that level, and you're susceptible, then it's a risk.

Risk belongs to you specifically, not to the substance in isolation.

This is where most non-toxic conversations get stuck. It's much easier to argue about a single concept (hazard, exposure, dose) than to walk through all six. This framework is the harder, slower path, and it's also the one that gets you closest to a real answer.

Recap