The warm spices in gingerbread, the woodsy aroma of pine and fir trees, and the fruity tang of mulled wine are smells synonymous with the holiday season. Many people enjoy lighting candles, incense and fireplaces in their homes to evoke the moods associated with these festive fragrances. Burning scented products may create a cozy ambiance, and in the case of fireplaces, provide light and heat, but some experts want people to consider how doing so contributes to the quality of the air indoors...