Ingredient quality makes a real difference: natural waxes, cotton wicks, and naturally-derived fragrance materials generally produce far fewer by-products than paraffin, synthetic oils, or metal-core wicks. That said, no flame is entirely neutral to the air around it: any burning candle introduces particles and compounds into the air. As with any home fragrance product, we recommend ventilating the room regularly, such as opening a window for a few minutes after use, and avoiding prolonged, continuous use in small, enclosed spaces.