Burning citronella candles indoors is not recommended, according to Life123. The scent emitted by citronella can be very difficult to remove from indoor fabrics, and the candles emit a secondary organic aerosol particulate into the air which can cause difficulty breathing and headaches.
Citronella candles can be an effective mosquito repellent when used outside as directed. The span of their protection varies depending on the size of the candles being used, but two, 2-inch-wide candles can cover an area of up to 6 feet.
The oil in a citronella candle comes from several different species of the lemongrass plant. These plants are native to Asia and India, and their oil is common in soaps, candles and perfumes. Citronella can even be used to flavor some foods.