When you turn on a humidifier, you might notice your air quality monitor showing worse readings and running harder. Curious about this, I looked into how humidifiers affect the air particles. Here are two key reasons why this happens.

1. Humidity Inflates Air Quality Readings

The most common air quality monitors work by sensing tiny particles in the air. Here’s the air quality monitor in a Coway air purifier that I took apart.

What you’re seeing is a small box with a laser in it. On the other side from the laser is a little sensor that detects how much of the laser light is reaching the sensor.

If you use a humidifier or take a shower, the air gets more humid. That humidity causes particles to swell and get bigger.

That’s important for air quality monitors because they detect particles of a certain size and above. For some particle counters, that’s 0.3 microns and above.

See how there are a lot of particles just to the left of the red line? When the air gets humid, those particles become just big enough to cross the threshold to be counted.

So when you take a shower, your air quality monitor will tell you you have more particles in your air—worse air quality. But you didn’t get more particles in the air. It’s just that some particles became large enough to count.

2. Some Humidifiers Will Put Minerals from the Water into the Air

Some “cool mist” and ultrasonic humidifiers release minerals from tap water into the air.

When researchers tested these kinds of humidifiers with the lungs of mice, they found it caused gene dysregulation and other problems. That’s why the US Environmental Protection Agency recommends using distilled water, which has no minerals in it. Or you can use types of humidifiers that boil water.