MILWAUKEE (AP) — As wildfire smoke from Canada continues to affect air quality across parts of the United States, health experts say keeping indoor air clean can help reduce exposure to harmful particles.

The biggest health concern isn’t the visible haze but the tiny particles in wildfire smoke, which can contribute to breathing problems, headaches, eye irritation and other short-term health issues. People with asthma, heart or lung disease, older adults and young children may be especially vulnerable.

Experts recommend these steps to improve indoor air quality during smoky conditions:

• Use central air conditioning if available. Replace HVAC filters on schedule and use the highest-rated filter your system can handle, ideally a MERV 13 or higher. Set the system to recirculate indoor air and close outside air intakes if possible.
• Run an air purifier. A portable purifier with a HEPA filter can help remove fine particles from the air. If you only have one unit, place it in the room where you spend the most time.
• Consider a DIY air filter. The U.S. Environmental Protection Agency says a box fan paired with MERV 13 furnace filters can effectively reduce smoke particles indoors when assembled properly.
• Seal your home. Close windows and doors and seal gaps where outside air may enter. Avoid activities that create indoor pollution, including burning candles, using fireplaces or cooking on a gas stove when smoke levels are high.
• Use window air conditioners correctly. Set window or portable air conditioners to recirculate indoor air rather than bringing air in from outside. If your unit doesn’t filter fine particles, experts recommend using it alongside an air purifier if possible.

If your home becomes too hot or smoky, officials recommend spending time in public buildings with clean, air-conditioned spaces, such as libraries or shopping centers.

Wildfire smoke exposure can cause coughing, shortness of breath, chest tightness, headaches and irritation of the eyes, nose and throat. Anyone experiencing severe breathing difficulty or other emergency symptoms should seek medical care immediately.