Spoiler alert: This may be why mold is growing in your home.

Although many people shut their windows and seal their homes in the winter to keep the cold air out and the warm air in, this move may be hazardous for your home and health.

“The data shows a strong correlation between Brits making an effort to reduce ventilation in their homes in a bid to save energy and a spike in instances of mold,” Nicholas Donnithorne, UK technical services manager at the water-proofing firm Peter Cox, told The Mirror this week.

He shared that three-quarters of Brits reduce ventilation in their homes to keep costs down and their homes heated, but the trapped air can cause mold to fester.

“We understand it can feel like a catch-22 — we all want to find ways to keep our energy bills down over the colder months, but trapping the warm moist air inside can inadvertently cause issues with mold later down the line,” Donnithorne explained.

Ventilation is important because it allows warm, moist air to move through a home instead of gathering condensation and creating mold.

Mold is most likely to grow in places where moisture collects, such as around leaks or floods, and on paper products, cardboard, ceiling tiles, and wood products.

It can also grow in dust, paints, wallpaper, insulation, drywall, carpet, fabric, and upholstery.

Donnithorne warns that “mold not only looks unsightly but can have implications for our health.”

Mold exposure can lead to a stuffy nose, wheezing, and red or itchy eyes or skin, according to the Centers for Disease Control and Prevention.

Donnithorne suggests using a dehumidifier to suck the moisture out of the air.

“If the issue persists, damp will rarely ‘sort itself out,’ so it is imperative to get professional help rather than just painting over it,” he said.