Winter Indoor Air: Humidity, Condensation, Smoke and Filters

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Winter changes the air inside a Bay Area home in ways people notice without knowing why: static shocks and a scratchy throat one week, streaming windows and a musty closet the next, wood smoke drifting in from down the street on a still night. The heat runs more, the windows stay shut, and whatever is in the house stays in the house. This page covers humidity (too little and too much), window condensation and mold, the Air District's winter no-burn rules, filters, and the carbon monoxide check every home with gas heat should have.

It is part of our Bay Area Winter Heating Help Center. For the year-round picture, including wildfire smoke and ventilation, see our Bay Area indoor air quality guide.

Humidity: aim for 30 to 50 percent

The EPA's guidance is to keep indoor humidity "between 30 and 50 percent" (EPA), and below 60 percent at most to discourage mold (EPA). That range matters more than any feeling of "dry" or "damp," so the first step costs very little: buy a digital hygrometer (a humidity meter, often built into a cheap indoor thermometer) and read it in a few rooms, morning and evening, for a week.

Bay Area winters are the rainy season, so don't assume your home is dry because the heat is on. Heated indoor air can feel dry, especially in a house that runs the furnace a lot and is well sealed. But plenty of local homes read on the damp side in winter, with a crawlspace under the floor, showers running, laundry drying indoors and windows closed against the cold. The hygrometer tells you which problem you actually have. The fixes point in opposite directions.

Our guide to humidity and your HVAC system explains how moisture changes how warm a room feels at the same thermostat setting.

Humidifiers: when they help and when they hurt

The EPA describes the upside plainly. Humidifiers relieve "dry nose, throat, lips, and skin" and help with "static electricity, peeling wallpaper, and cracks in paint and furniture." It also warns that excess moisture can encourage biological growth in the home, and it recommends water "with low mineral content, such as distilled water" and cleaning portable units "every third day" (EPA).

• Pros: real comfort when the meter reads low, quick relief in one bedroom with a portable unit, and a whole-house humidifier on the furnace can cover every room.
• Cons: easy to overdo in a damp climate, a dirty tank or pad grows things you then breathe, tap water leaves mineral dust, and a furnace-mounted humidifier adds a water line and a drain that need yearly attention and can leak.
• Rule of thumb: run one only while the hygrometer is under 30 percent, and turn it off before it passes 50. If windows start to fog, it is already too much.

Window condensation and mold in a wet winter

Condensation forms when warm, moist room air touches a surface colder than its dew point. On a cold Bay Area night that surface is the glass, especially older single-pane windows and aluminum frames, and the corners of outside walls behind furniture. A little fog at the bottom of a window on the coldest mornings is common. Water running onto the sill every day, black spots on the frame or caulk, or a musty closet on an outside wall means the house holds more moisture than it can shed.

1. Run bathroom fans during showers and for a while after, and make sure they vent outdoors rather than into the attic.
2. Use the range hood when you boil or simmer, again vented outdoors if yours is.
3. Dry laundry in a vented dryer, not on racks indoors, during the wettest weeks.
4. Wipe sills dry in the morning and pull furniture a few inches off cold outside walls so air can reach them.
5. Keep the heat reasonably steady. A house that swings cold overnight gives moisture more cold surfaces to land on. Our cold rooms and uneven heating page helps if one room is always the damp, chilly one.
6. Find and fix the water. A roof or plumbing leak, a wet crawlspace or a clogged gutter feeding a wall will beat any amount of ventilation.

The EPA's target is to keep indoor humidity below 60 percent, ideally between 30 and 50 (EPA). Mold on a large area, mold that keeps coming back, or mold inside ducts or the furnace cabinet is a job for a professional, not bleach.

Wood smoke and the Air District's no-burn rules

A fire in the fireplace feels like the obvious fix for a chilly evening, but on many winter nights it is illegal in the Bay Area. On a Spare the Air alert day for fine particles, it is "illegal indoors and outdoors throughout the Bay Area to burn wood, fire logs, pellets, or other solid fuels" in a fireplace, wood stove, fire pit or other wood-burning device (Spare the Air). Alerts are most common November through February, and a no-burn alert can be called any day of the year.

The rule got stricter on 1 October 2025. The Air District lowered the forecast level that triggers an alert from 35 to 25 micrograms per cubic meter of fine particulate, and estimated 19 to 41 alerts a year compared with a historic average of 15 (Bay Area Air District). Expect more no-burn nights than you are used to.

• Fines: a first-time violator is encouraged to take a wood smoke awareness course or receives a $100 ticket. A second violation brings a $500 ticket, and the amounts go up after that (Bay Area Air District).
• Exemption: only for homes with no permanently installed heating, where a wood stove or fireplace is the sole source of heat. The device must be EPA-certified or pellet-fueled and registered with the Air District. "An open-hearth fireplace does not qualify" (Bay Area Air District), and registration is required to be exempt (Bay Area Air District).
• How to check before you light: text START to 81757 for alerts, call 1-877-4NO-BURN (1-877-466-2876), check sparetheair.org or baaqmd.gov, sign up for email AirAlerts, or use the Spare the Air app (Bay Area Air District).

Smoke from a neighbor's chimney also gets indoors through gaps and fresh-air paths. On a smoky, still night keep windows closed, run the furnace or heat pump fan to recirculate through a good filter, and avoid kitchen or bath fans longer than needed, since they pull outdoor air in through cracks to replace what they exhaust. If your home really has no other heat, that is a heating problem worth solving; our guide to furnace rules and heat pumps covers the options.

Filters: check monthly, change at least every 3 months

ENERGY STAR's advice is to check your filter every month, especially in winter and summer, change it if it looks dirty, and "at a minimum, change the filter every 3 months" (ENERGY STAR). Winter is when the system runs longest with the house closed up, so the filter is doing most of the work of keeping dust, dander and smoke particles out of the air you breathe.

A higher MERV rating catches finer particles but also resists airflow more. A filter your system cannot push air through can starve the furnace and the far rooms. Check what your system is designed for before jumping to a thick, high-MERV filter; our HVAC filter guide for Bay Area homes walks through MERV ratings, sizes and how to tell whether your ducts can handle an upgrade.

Carbon monoxide: the winter air check that matters most

Every gas furnace, wall heater, water heater and fireplace is a possible carbon monoxide source, and the risk peaks when heating runs most. California's public health department says CO deaths "occur most frequently during cold winter months" and recommends having a qualified technician inspect your furnace every year (CDPH).

• Know the symptoms: "headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion." People who are asleep "can die from CO poisoning before they have symptoms" (CDC).
• Have working alarms: California requires a CO device in every home with a fuel-burning heater or appliance, a fireplace or an attached garage (Health and Safety Code 17926). The CPSC says to put CO alarms on every level and outside sleeping areas (CPSC). Test them when you first turn the heat on for the season.
• If an alarm sounds or symptoms appear: get out first, then call 9-1-1.

A yearly heating tune-up is when a technician checks the burners, venting and heat exchanger that keep combustion gases out of the house. Our fall and winter services cover furnace and heat pump maintenance for homes across the South Bay, Peninsula, Tri-Valley and East Bay.

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Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.