Candles that contain oil of citronella are a favorite choice for keeping mosquitoes at bay outdoors, but burning them indoors might not be a good idea for people with lung or breathing issues such as asthma. If you're healthy overall, the candles won't cause a problem, but burning citronella, fragrant candles and even incense can result in indoor air-quality issues, especially for the young, elderly and those with breathing problems.
In 2001, the Environmental Protection Agency released a report, "Candles and Incense as Potential Sources of Indoor Air Pollution", which indicated candlewicks with lead cores leave harmful lead residue in the air after being burnt above threshold recommended by the agency. Although most U.S. manufacturers stopped making candles with metal wire cores in 1974, imported candles may contain wicks with lead or other metals such as tin or aluminum, which emit harmful fumes. Instead, choose candles that use natural cotton or hemp wicks.
Citronella and other candles that don't burn right emit soot that can cause damage to ceilings, walls and floors. Conditions that cause candles to emit soot include placing candles in drafty areas, the materials used in the candle itself and an improper wick length for burning. Not all candles produce soot, as some produce more than others do based on the candle and wick's ingredients. The EPA discovered in 2001 that some, not all, candles can emit organic chemicals such as acrolein, formaldehyde and acetaldehyde that can exceed safe thresholds.
Natural Ingredient Candles
You don't need to burn citronella candles indoors unless you leave screenless windows open in the summer. These candles work better outdoors to keep mosquitoes at bay. When you burn candles indoors, choose candles made from hard natural waxes, not candles in glass containers, as these are soft wax candles often made from paraffin – a petroleum product -- which can emit harmful toxins benzene and toluene. In homes that have family members with breathing issues, burn beeswax, palm or soy-based candles made from natural ingredients that don't produce toxins.
Burn Candles Safely
Don't burn imported candles with a lead core or wire wickas these can create indoor quality issues and leave harmful toxins in the air. Choose candles with single, thin wicks trimmed to 1/4 inch before lighting. While you may prefer aromatic candles, many of these contain scents not meant for use in a candle, some of which may contain volatile aromatic hydrocarbons. If the candle leaves a sooty residue on surrounding surfaces or on the candleholder, stop burning the candle as it may contain harmful ingredients.