Benzene is a ring-shaped molecule that's known to be harmful to health. It's found in petroleum products, and it's used to make lots of everyday materials such as plastics and synthetic fibres. Small amounts of it are found in the smoke and vapour from traditional and e-cigarettes, as well as in the smoke that's given off when you burn candles.

Xylene is chemically similar to benzene, and exposure to too much of it is linked to various health conditions. In the home, it's found in paints and varnishes, and it may also be present in some pesticide products such as flea sprays. It's also found in the smoke and vapour from cigarettes and e-cigarettes.

Formaldehyde is an excellent preservative, which means it prevents things from rotting. Exposure to too much of it can have negative health effects. It's particularly used to make building materials, especially pressed-wood materials like plywood, and it also turns up in glues, insulation materials, cigarette smoke and some types of hair products.

Ammonia is an alkaline chemical that's known to be irritating to your skin and lungs. It's used in cleaning products because it's good at breaking down grease, and because it evaporates quickly, leaving surfaces streak-free. Unfortunately, that also means it ends up in the air. It's also found in some hair dyes and hair straightening mixtures.

Trichloroethylene (TCE) is a solvent, which means it's used to dissolve other things. Regular exposure over a long period of time has been linked to some serious health conditions. In the home, it's mostly found in glues, varnishes, paint and stain removers, as well as in cigarette smoke.