A recent study at South Carolina State University found that unscented, uncolored paraffin candles emitted chemicals including toluene and benzene, while beeswax- and soy-based candles did not. Candle wicks can also create air pollution: metal-core wicks may contain lead, which becomes airborne upon burning. The CPSC banned lead-core wicks in 2003; to avoid lead, do not burn older candles.
All candles produce soot, fine particles which may irritate the lungs. Scented candles contain additional chemicals which increase emissions of both soot and chemicals. To reduce soot, choose unscented candles, keep the wick trimmed, and minimize drafts; further recommendations may be found here. To reduce emitted chemicals, choose unscented candles, and burn candles infrequently or choose soy or beeswax candles.