Did you know that humidity is actually a huge indicator of the health of your home? According to Aviad Elgez, N.D., a naturopath focused on environmental toxicology, if it doesn't fall into the 40 to 50 percent range, you run the risk of breathing air that is either too dry, which could cause respiratory irritation, or too damp (hello, mold).

To ring in the holiday season and the start of lots of snuggly time indoors with friends and family, we're breaking down the basics of healthy moisture levels at home. And no, we won't judge you for putting a humidifier on your Christmas list this year.

Most homes have a humidifier in the furnace, which should give you an idea of humidity—but only in that one area. Humidity levels will change, however, depending on what room you're in due to factors like insulation and air flow from windows and fans. That means it's a good idea to invest in a portable humidity meter that you can carry around your whole home. It'll only set you back around $10 to $20, and you can find them online and in most home goods stores.

When you've got your meter and are measuring humidity in each room, don't forget about the bathroom! Elgez says that while most bathrooms have built-in ventilation, or fans, these days, the ones that don't are poised for high humidity and mold growth. "I've had many patients getting sick living in an apartment that doesn't have a fan or window in the bathroom," he says. "They would complain it was very damp and moldy."

According to Elgez, once you hit a humidity level of 60, you run the risk of getting mold, and just because you don't see it doesn't mean it's not there.

"A lot of people think black mold is the one to worry about, but it turns out there are a lot of different species of mold that could grow at home," he says. "White mold, Aspergillus, can actually be more toxic depending on what species it is." While black mold requires about 90 percent humidity to grow (that's why you'll usually see it in eternally damp places like the shower or windowsill), white mold, which you usually need a microscope to see, can survive at about 60 to 70 percent humidity.

But don't freak out: If a room's humidity is approaching the danger zone, a small dehumidifier will probably do the trick to bring it back down to normal levels. There are tons of options on the market these days; just keep in mind that some are noisier than others if you're placing it in a bedroom.

If you're concerned that your home has a mold problem, either because you can see it growing or you are experiencing symptoms of mold exposure like headaches and chronic fatigue, you're going to want to call in an expert to test the air quality.

"Air sample testing is one of the best things you can do—that's when they look for spore counts," Elgez explains. "It's a little tricky, so you really want to make sure it's done properly. If you have a positive spore count, there's a species growing there." Pro tip: If a room has that musty scent that you'd associate with an old basement, Elgez says it likely has mold: "Nothing really produces that smell. It's a big red flag."

Having eternally dry air at home isn't dangerous, but it's just plain uncomfortable and could lead to respiratory irritation, nose bleeds, dehydration, and the like. If your air clocks in at under 40 percent, it's time to look for a humidifier. Elgez recommends staying away from ones that use hot water because they're more likely to attract mold, recommending Sonic cold-air models to his patients instead

Once you pinpoint rooms that tend to fall outside the recommended range, you can balance out their humidity by adopting a simple habit: Open the windows after it rains to bring humidity levels up, or do it on a drier day and watch them go down. This is a nice routine to get into for overall air quality too (as long as you don't live in a super-polluted area), since outdoor air actually tends to be cleaner1, and maintaining a steady airflow will help keep things fresh at home.