It’s easy to think of some pretty nasty substances. Botulinum toxin, a poison produced by Clostridium botulinum bacteria, is the most toxic naturally occurring substance on Earth, blocking nerve signals to muscles to cause death by paralysis. Similarly, the potent nerve agent VX, developed as a chemical weapon by the British military, also asphyxiates its victims by paralyzing the respiratory muscles. Chlorine trifluoride, an ultracorrosive colorless gas, is so reactive that it spontaneously explodes on contact with seemingly innocuous materials like water, sand and even the ashes of substances which have already burnt.

There are so many diabolical possibilities, but which chemical is the most dangerous? It comes down to a combination of effect and exposure—how much makes a deadly dose and what exactly will it do to you? Nerve agents are widely considered the most toxic chemical weapons owing to their tiny toxic limits and devastatingly rapid impacts on the human body: Just 10 milligrams (that’s ten thousandths of a gram) of VX is enough to cause death within minutes. Yet just one person has been killed by the nerve agent over the last decade.

Meanwhile, more than 100,000 people are accidentally poisoned in the U.S. every year by common household chemicals such as bleach and disinfectant, even though these substances are slower-acting and far less toxic than VX. And some common chemicals can be fatal when combined. For instance, combining drain cleaner and bleach will release poisonous chlorine gas. Those two examples highlight a key problem in ranking chemicals in order of danger: To evaluate danger, you need to know how likely you are to encounter a chemical.

Safety professionals define danger using a combination of two factors: hazard and risk. “A hazard is something with the potential to cause harm. Risk is the likelihood that harm will arise and the severity of that harm,” said Richard Webb, the health, safety, environment and well-being officer at the University of Cardiff’s School of Chemistry. The hazard is therefore a fixed property of a tool or chemical, while the risk varies depending on how that object is used. We automatically consider this balance of factors every day. Take the example of a kitchen knife: We know the blade is sharp and will cut things, including us, in the right circumstances. But it’s how we use and store the knife that determines whether it poses a danger to us, Webb told Live Science. This same logic applies to chemicals. “Even a very hazardous chemical does not pose any risk if there is no exposure,” a spokesperson for the Finland-based European Chemicals Agency told Live Science. Botulinum toxin, VX and chlorine trifluoride are therefore extremely hazardous but very, very low risk to the average person.