Like many holiday traditions, lighting candles and fireplaces is best done in moderation

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. All flames release chemicals that may cause allergy-like symptoms or contribute to long-term respiratory problems if they are inhaled in sufficient quantities.

However, people don't have to stop sitting by the hearth or get rid of products like perfumed candles and essential oil diffusers, said Dr. Meredith McCormack, director of the pulmonary and critical care medicine division at John Hopkins University's medical school. Instead, she recommends taking precautions to control the pollutants in their homes.

''Clean air is fragrance free,'' said McCormack, who has studied air quality and lung health for more than 20 years. ''If having seasonal scents is part of your tradition or evokes feelings of nostalgia, maybe think about it in moderation.''

What to know about indoor air quality

People in the Northern Hemisphere tend to spend more time indoors during the end-of-year holidays, when temperatures are colder. Indoor air can be significantly more polluted than outdoor air because pollutants get trapped inside and concentrated without proper ventilation or filtration, according to the American Lung Association.

For example, active fireplaces and gas appliances release tiny airborne particles that can get into the lungs and chemicals like nitrogen dioxide, a major component of smog, according to the U.S. Environmental Protection Agency. Cleaning products, air fresheners and candles also emit air pollutants at varying concentrations.

It is also important to note that some pollutants have no smell, so unscented products still can affect indoor air quality, experts say.

Some people are more vulnerable

Polluted air affects everyone but not equally. Children, older adults, minority populations and people of low socioeconomic status are more likely to be affected by poor air quality because of either physiological vulnerabilities or higher exposure, according to the environmental agency.

Children are more susceptible to air pollution because of their lung size, which means they get a greater dose of exposure relative to their body size, McCormack said. Pollutants inside the home also post a greater hazard to people with heart or lung conditions, including asthma, she said.

Signs of respiratory irritation include coughing, shortness of breath, headaches, a runny nose and sneezing. Experts advise stopping use of pollutant-releasing products or immediately ventilating rooms if symptoms occur.

Practical precautions to take

Ellen Wilkowe burns candles with scents like vanilla and cinnamon when she does yoga, writes or when she is showering at her home in New Jersey. Her teenage daughter, on the other hand, likes more seasonally scented candles like gingerbread.

''The candle has a calming presence. They are also very symbolic and used in rituals and many religions,'' she said.

Wilkowe said she leans toward candles made with soy-based waxes instead of petroleum-based paraffin. Experts note that all lit candles give off air pollutants regardless of what they are made of.

Buying products with fewer ingredients, opening windows if the temperatures allow, and using air purifiers with HEPA filters are ways to reduce exposure to any pollutants from indoor fireplaces, appliances and candle displays, McCormack said. She also recommends switching on kitchen exhaust fans before starting a gas-powered stovetop and using the back burners so the vent can more easily suck up pollutants.

''Small improvements in air quality can have measurable health benefits," McCormack said. "Similarly to if we exercise and eat a little better, we can be healthier.''

Rachael Lewis-Abbott, a member of the Indoor Air Quality Association, an organization for professionals who identify and address air quality problems, said people don't usually notice what they are breathing in until problems like gas leaks or mold develop.