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Too much moisture or high humidity can promote the growth of molds and mildew.  Mold and mildew can be harmful to the respiratory system and may cause respiratory problems.   Devices to measure humidity are widely available at any home improvement or household stores. Purchase and use a dehumidifier if your humidity levels are too high.  Remember to regularly empty the water it collects from your house. Stagnant air can be harmful to breathe as it may have accumulated allergens, microbes, or other irritants.  Create airflow in your house to let fresh air enter your space and allow stale air an exit.  Opening a few windows is the easiest way to create airflow in your house. You may also try to place a fan, either pulling air in or pushing air out, to help increase airflow. HVAC systems may be installed in your home to greatly improve air quality. Smoking indoors will immediately introduce harmful chemicals into the air.  These won't simply vanish with the smoke but will instead cling to many surfaces in the room.  If you must smoke, do it outside.  Smoking anywhere will harm your lungs and make breathing less efficient. Incense or other scented combustibles also release smoke and reduce the air quality in your home. House plants are a great and easy way to improve the quality of air in your house.  Plants absorb many chemicals in the air that humans cannot breath, such as carbon dioxide, and release oxygen that we need.  Some plants are more efficient at this than others, review the list and pick your favorites:  Aloe vera. Spider plants. English ivy. Azaleas.
Monitor moisture levels in your house. Make sure your house is properly ventilated. Avoid smoking. Get some house plants.