Winter is heating season, and when we run the heat in our homes we tend to keep our windows and doors shut, to help keep the heat in and the cold out. (Makes sense.) But what we’re also keeping out is fresh air.

Keeping your windows and doors shut over winter helps save energy, not to mention money, too—all pluses. But if you never exchange the air inside your home for fresh air, it can get stale and potentially lead to health issues.

For example, some people might have heard of sick building syndrome, or SBS for short. That’s when you spend a lot of time inside a space that is tightly sealed and with little ventilation, like your home.

Not only can this cause condensation issues inside your house (i.e., weeping windows), which we know can lead to mould, but it also allows toxins already inside your home to build-up. That includes things like VOCs (volatile organic compounds), mould spores, dust, smoke, radon, viruses and bacteria. Breathing in these things over an extended period of time isn’t good for your health.

For example, it can make you feel sick, tired and drowsy; cause headaches, dizziness and nausea; irritate your eyes, nose and throat; it can even lead to things like building-related illness, or BRI. Symptoms of BRI include fevers, coughing, muscle aches, and tightness in your chest.

Some people might think that their furnace is bringing fresh air into their home, but that’s not always the case.

A furnace has two vents: One that exhausts old air inside the house outdoors, and the other for air intake. Older furnaces—say about 12-15 years old and older—don’t bring in fresh air from outside. The air intake is located inside the house (usually a vent sticking out of the furnace) so it’s actually drawing air from inside the home.

What about ceiling fans?

A ceiling fan circulates the air already inside your home; it doesn’t bring fresh air in. We all know hot air rises, so a ceiling fan can help circulate the warm air that tends to hang around your ceiling, which can help save energy, but it doesn’t help when it comes to bringing fresh air into your home.

How do you bring fresh air inside?

The best solution is to get an HRV or heat recovery ventilator. Some reputable homebuilders nowadays will install an HRV unit in their new homes because they’re building them to be more airtight and energy-efficient. So to prevent problems like condensation, mould and poor indoor air quality they will automatically install an HRV.

An HRV brings in fresh air from outside, and preconditions it to the temperature inside the home using the air that’s going out. This system recovers up to 88 percent of the heat, and uses a minimal amount of electricity to do it thanks to an electronically commutated motor (ECM).

If you want to bring in fresh air over the winter but you can’t invest in an HRV just yet, don’t be afraid to open your windows a bit during the day. But do it on warmer winters days, like when it’s a balmy five degrees outside. And while you have the windows open, run the main exhaust fan in your home, which is usually located in the main bathroom, to help exhaust old, stale air.

You don’t need to do this for hours; 15 to 20 minutes is enough to make a difference. It’s also a good solution for homes that don’t have forced air. Yes, you will be losing some energy, but the health benefits you get from bringing fresh air into your home can offset this energy loss.

The air inside your house isn’t just a home issue. It’s a health issue.

Remember to change your furnace filter at least once every 3 months—once a month during the cold season. And if you and/or your family are experiencing symptoms related to poor indoor air quality, such as SBS or BRI, I would get a healthy home inspection that includes indoor air sampling, because when it comes to the air you breathe, you shouldn’t take any chances.