Indoor air and dust at home: where dust gets in, humidity and odours, and MERV filters

In our climate dust is a constant visitor, and humidity appears in coastal cities and around bathrooms and air conditioners. The U.S. EPA says there are three basic ways to improve indoor air: source control, ventilation and air cleaning, and that removing a source or reducing its emissions is usually the most effective1.

1) Where does dust get in?

Practical check (advice, not a regulation): look for lines of dust at these places after a sandstorm.

• Windows: seals around the sash, drainage slots in sliding windows, and the gap between frame and wall. See window problems.
• Doors: the threshold and gaps around the frame.
• Service penetrations: around AC and electrical pipes that pass through the outer wall.
• Exhaust fans and vents: without a cover or backdraft damper.
• False ceilings and roof rooms: unsealed openings to outside.

Seal with materials suited to the spot: silicone or sealant for small joints (see sealants and joint fillers and resealing silicone), and weatherstripping for doors and windows.

2) AC filters: what does MERV mean?

• The MERV rating reports a filter's ability to capture particles between 0.3 and 10 microns, on a scale of 1 to 162.
• If you upgrade a central system filter to a higher efficiency, EPA recommends at least MERV 13, or as high as the system fan and filter slot can accommodate; ask an HVAC technician about the highest efficiency your system can take2.
• All filters need periodic replacement per the manufacturer's instructions2.

Portable air cleaners: effectiveness depends on how much of the pollutant they capture and how much air they pass through the filter1. Choose one sized for the room.

3) Humidity, mold and odours

• Indoor humidity: below 60%, ideally 30–50%3. A cheap hygrometer is enough to monitor it.
• If wet materials are dried within 24–48 hours of a leak, in most cases mold will not grow3.
• The key to mold control is moisture control; cleaning without fixing the water source brings the problem back3.
• Keep AC drip pans and drain lines clear, use exhaust fans when showering and cooking, and vent moisture-producing appliances outside3.

4) Ventilation

Ventilation means bringing in outdoor air to dilute indoor pollutants1. On dusty days this conflicts with keeping dust out, so:

Practical tips: run bathroom and kitchen fans during use, air the house on clear days, close windows during sandstorms, and make sure outdoor-air intakes on central systems have filters that are cleaned.

5) Quick checklist

1. Lines of dust at windows and doors? Seal the gaps.
2. AC filter dirty or old? Clean or replace it.
3. Room humidity above 60%? Find the source and improve extraction.
4. Mold spots or a damp smell? Fix the water source first, then clean.
5. Water pooling under the AC or in its pan? Clear the drain line.