Volatile organic compounds: something's in the air
You can't see them, but they're all around us. They aren't listed as ingredients on the objects we bring in our home, but they're often there. They're volatile organic compounds, or VOCs, a wide range of carbon-based molecules (organic compounds) used in a wide range of products that find their way into our homes. Under normal conditions, they vaporize, effectively leaving their host and entering the air (that's the "volatile" part) where they combine with other airborne compounds to form ozone, which isn't good to breathe.
Though they exist everywhere in the environment -- the most common volatile organic compound is methane, which comes from everything from wetlands to cow farts and other ruminant gases to rice agriculture -- they are most well-known for the harm they can cause indoors, where they can be introduced via paint, carpets, furnishings, and cleaning agents.