Response: In addition to leaving a nice smell, scented candles can also leave black soot and other pollutants (such as formaldehyde and acrolein) inside your home. The soot (especially) can deposit onto your walls, ceilings, and other indoor surfaces, leaving these surfaces looking dirty and dingy.
Not all candles are equally polluting. Several scientific studies have shown scented candles to be more polluting than non-scented varieties, producing more soot as the result of more incomplete combustion that may occur from the presence of additional chemicals in the candle. The amount of soot produced can vary depending not only on the type of candle, but also the type and length of the wick and how drafty your house is. This variation can cause one type of candle to emit 100 times more soot than another type of candle. As you are probably seeing, emitted soot can mean more cleaning and interior home maintenance for you. In addition, it is also possible that soot and other chemicals emitted from the candles may harm your health, since soot from outdoor combustion sources has been linked to bad health outcomes. However, the health impacts from candle-produced soot are not known.One type of candle that has been linked to bad health outcomes is candles with lead core wicks, which are used to make the wicks stand up and to prevent them from falling in the candle wax. Several scientific studies have shown candles with lead wicks to emit lead when burned, often resulting in indoor lead levels higher than EPA-recommended limits, making leaded wick candles an important public health concern. As a result of these concerns, lead-containing wicks have not been used in candles made in the U.S. since 1974; however, imported candles with lead wicks can still be bought in the US and other countries. Clearly, you should avoid buying these candles. As a precaution, you could avoid all candles with shiny metal wire inside the candle wicks. [As a note, some candles with shiny metal wires do not contain lead but contain zinc or tin, which have not been linked to added health risks.]
In addition to avoiding candles with lead containing wicks, you may also want to take other steps to minimize pollutant emissions from candles. For example, you should keep wicks trimmed to one-quarter inch and keep candles out of the wind or other drafts, both of which will allow for more complete combustion. You should also consider switching from scented to non-scented candles, especially to those that are made from beeswax or other natural materials. Finally, you should only use candles that burn cleanly (without the tell-tale trail of black smoke from your candle) and use a snuffer to put out the candle.
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Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to Helen@TreeHugger.com (please use a descriptive email subject line and mention if you want to remain anonymous or not).