Air-conditioners can save lives and keep us comfortable on blistering days. But they may also carry health risks — namely, those related to mold.

This was highlighted again last week, when the manufacturer Midea voluntarily recalled about 1.7 million air-conditioners because of a design flaw that encouraged mold growth.

While that flaw made mold growth more likely, any air-conditioner can get moldy, no matter how well designed it is. Fortunately, there are ways to reduce the risk.

Why do air-conditioners get moldy?

It’s simple: moisture.

Whether you have a window unit or central air, the basic process is the same. Air-conditioners work by repeatedly converting a refrigerant from gas into liquid, and back to gas. As the refrigerant evaporates, it absorbs heat from the air via a cold component called the evaporator coil. A fan blows over the coil, pushing cool air into the home, while hot air is pulled back outside and the refrigerant is condensed back into liquid to restart the process.

As warm air cools, it produces condensation. Think of the way water droplets form on the outside of a glass when you pour cold water into it. In air-conditioners, a drain pan collects this water, and well-designed systems allow it to flow outside rather than remaining stagnant. But those systems can malfunction, or simply be imperfect.

That means moisture may sit inside your air-conditioner for an extended time, leading to mold growth, said Mark Mendell, a former air pollution research specialist at the California Department of Public Health and former staff scientist and epidemiologist at the Lawrence Berkeley National Laboratory.