Imagine an ordinary Tuesday. You wash your hair, put on deodorant, drink coffee, pack lunch in a plastic container and commute through traffic to get to work. At work, the custodial staff wipes down a shared table with disinfectant. At home, you cook dinner, clean the kitchen and run the dishwasher.

Each of these ordinary moments can involve exposure to chemicals. By itself, that is not a reason for concern. After all, chemicals make up the entire physical world.

But depending on the dose, timing and circumstances of exposure, some chemicals in our environment – both naturally occurring and human-made ones – may affect health.

Most everyday exposures occur at low levels, and many products are designed and regulated with safety in mind. But as a board-certified toxicologist who studies how chemical exposures affect human health, I rarely ask whether a single chemical is safe in isolation.

A more realistic question is: What might the health effects be when many low-level exposures overlap?

Mixtures are the rule, not the exception

Decades of research on the health effects of individual chemicals have helped scientists identify hazards, estimate safe exposure levels and develop regulations.

But the challenge is that people are rarely exposed to one chemical at a time.

Air is a changing mixture of particles, gases and vapors. Indoor air can differ from outdoor air because of cooking, cleaning, building materials, ventilation and chemical reactions that occur inside enclosed spaces.

Food often contains residues from multiple pesticides, as monitoring programs in Europe and the United States have identified. This reflects the fact that crops may be exposed to more than one pesticide during production.

Household products add another layer. Cleaning products, cosmetics and personal care products can contain fragrances, preservatives and other ingredients........

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