Citronella candles are burnt to repel bugs. The scent is derived from the oil of citronella grass which can repel mosquitoes and other flying insects. They can be burnt indoors in some cases to repel indoor bugs, but they can aggravate lung conditions.
The Environmental Protection Agency determined that citronella itself is not harmful to health. The only harm that is caused by citronella outside of candles is possible skin irritation. However, the EPA has stated that scented candles, such as citronella candles, can be harmful to the health when burnt.
When burnt, citronella candles disperse tiny soot particles into the air, according the EPA and the American Chemical Society. This can result in the same health problems caused by other types of scented candles.
Lead wicks have been found to cause many of the health problems caused by the indoor burning of scented candles, according to a 2001 EPA project study on candles and incense. The U.S. has had a ban on lead wicks since 2003.
Asthma, lung cancer and other lung problems can be aggravated by soot in the air when scented candles are burnt indoors because the soot travels deep into the lungs, according to the EPA.
Citronella is labelled by the EPA as a pesticide, but it is an unregulated one because of its low toxicity. Because it is considered a safe, natural pesticide by the EPA, citronella is only required to have protective labelling when it is included in a skin product.