To test for humidity, we burned cigarettes to pollute a sealed room…Then we turned on the Blast MiniHEPA air purifier for about 25 minutes. During the test, we used the Air Visual Pro laser particle counter to track particulate pollution and humidity. In 20 minutes, particulate pollution dropped from nearly 200 micrograms to zero (red line). But meanwhile, there was virtually no change in humidity (blue line).
For purifiers installed with a carbon filter, we used the Blast for our test because it has a massive carbon filter. With such a large carbon filter, we can detect even a small effect on humidity. The Blast cut particulate from 250 micrograms to nearly zero in 15 minutes. Meanwhile, humidity dropped 2% in about 20 minutes.
HEPA air purifiers have no effect on humidity. Carbon filters slightly decrease humidity.
To read more about our tests, click here: https://www.quora.com/Does-an-air-purifier-dry-out-the-air-in-your-home