The health evidence against plug-in and spray air fresheners is mounting; many emit chemicals and ultra-fine particulates that aren’t identified on the label. Some also contain terpene, a fragrant chemical that’s widely found in natural substances, such as pine resins.

But when confined inside a house, terpenes react with naturally occurring ozone in the air and form compounds that have long-term effects on the respiratory system (asthma, for example).

The alternative? Keep a clean house, use environmentally friendly cleaners, and get a whiff of pine scent while taking a nice long walk outside.