Air Quality
How to Improve Indoor Air Quality This Winter
March 26, 2026 · 9 min read
Key Takeaways
- Winter indoor air is often 2–5 times more polluted than outdoor air due to sealed homes
- Upgrading to a MERV 11–13 furnace filter captures 85–95% of airborne particles
- A whole-home humidifier maintains 30–50% humidity, reducing respiratory irritation
- UV-C germicidal lights kill up to 99.9% of mould and bacteria circulating through ductwork
In This Article
When temperatures drop below freezing, GTA homeowners seal their windows, crank the furnace, and spend 90% of their time indoors. The result? Air that recirculates the same dust, pet dander, cooking fumes, and volatile organic compounds (VOCs) with nowhere to escape. Here are eight proven strategies to breathe easier this winter without wasting energy.
Why Indoor Air Quality Drops in Winter
In summer, open windows provide natural ventilation. In winter, homes are sealed tight to conserve heat. This creates a closed-loop environment where pollutants accumulate. Common winter air quality culprits include:
- Dry air: Furnace-heated air drops to 15–25% relative humidity — far below the comfortable 30–50% range
- Dust and dander: Forced-air systems circulate particles through every room
- Carbon monoxide: Gas furnaces, stoves, and fireplaces produce CO that needs proper venting
- VOCs: Household cleaners, paint, and building materials off-gas into stagnant air
- Mould spores: Condensation on cold windows and walls feeds hidden mould growth
Upgrade Your Furnace Filter
The single most impactful change you can make is upgrading your furnace filter. Most homes come with basic MERV 1–4 fibreglass filters that catch large dust bunnies and little else. Upgrading to a MERV 11–13 pleated filter captures 85–95% of particles down to 1 micron — including pollen, mould spores, dust mite debris, and pet dander.
Change your filter every 60–90 days during heating season, or monthly if you have pets. A clogged filter restricts airflow, forcing your furnace to work harder and circulate fewer air changes per hour.
Important: Don't jump straight to MERV 16+ (HEPA-grade) filters without checking your furnace's specifications. High-MERV filters restrict airflow, and many residential furnaces can't handle the pressure drop. MERV 11–13 hits the sweet spot for most systems.
Control Humidity Levels
Ontario winters are brutal on indoor humidity. Furnace-heated air can drop below 20% relative humidity — drier than the Sahara Desert. This causes cracked skin, nosebleeds, static electricity, and increased susceptibility to respiratory infections. Low humidity also causes hardwood floors and furniture to shrink and crack.
A whole-home humidifier installed on your furnace ductwork maintains 30–50% relative humidity throughout the house. Flow-through (bypass) models cost $300–$500 installed, while steam humidifiers ($800–$1,500) offer more precise control and higher output for larger homes.
Improve Ventilation Without Losing Heat
You can't open windows in January without freezing, but you can improve ventilation mechanically. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) exchanges stale indoor air for fresh outdoor air while recovering 70–80% of the heat energy. The result: fresh air without the energy penalty.
Kitchen and bathroom exhaust fans also help — run your range hood while cooking and leave the bathroom fan on for 20 minutes after showering. These remove moisture and pollutants at the source, where they're most concentrated.
Consider UV Germicidal Lights
UV-C lights installed inside your ductwork or near the evaporator coil neutralize airborne bacteria, mould spores, and viruses as air passes through the system. They don't filter particles — they sterilize biological contaminants that filters miss. For a deeper look at whether this upgrade makes sense, see our guide on UV lights in HVAC systems.
Schedule Professional Duct Cleaning
If your ducts haven't been cleaned in 3–5 years, they may be harbouring dust, pet hair, construction debris, and even mould. Professional duct cleaning removes accumulated contaminants from the entire duct system, registers, and the air handler. Budget $300–$500 for a thorough cleaning of a typical GTA home.
Fair warning: many "duct cleaning" deals advertised for $99 are bait-and-switch operations. A legitimate cleaning takes 3–5 hours with truck-mounted vacuum equipment. If someone promises to clean your whole house in 45 minutes for $99, walk away.
Frequently Asked Questions
How often should I change my furnace filter in winter?
Every 60–90 days for standard households, or every 30 days if you have pets, allergies, or recent renovations. Hold the filter up to a light — if you can't see through it, it's overdue.
Do portable air purifiers help?
Yes, but only in the room where they're placed. A quality HEPA purifier effectively cleans a single room. For whole-home improvement, upgrading your furnace filter and adding a whole-home humidifier delivers better overall results at a lower per-room cost.
What humidity level should I maintain in winter?
Aim for 30–40% relative humidity when outdoor temperatures are below –10°C, and up to 45–50% during milder winter weather. Too much humidity causes condensation on windows and can promote mould growth.
Is a carbon monoxide detector enough for gas furnace safety?
It's essential, but not sufficient on its own. CO detectors catch dangerous leaks, but annual furnace maintenance prevents leaks from developing in the first place. A cracked heat exchanger is the most common source of CO from a furnace.
Breathe Easier This Winter
Improving indoor air quality doesn't require a full home renovation. Start with a better furnace filter, add a humidifier if your home is dry, and consider UV-C lights if allergies or respiratory issues are a concern. For a professional assessment, request a free quote or call 1-855-539-4328.