In the past five years, I’ve tested dozens of indoor air-quality monitors but have relied on my iPhone’s Weather app to check outdoor air quality. And the more I review air quality monitors, the more I appreciate the Zen’s easy-to-see color scale. There’s an option to have the monitor be public or private; by clicking public, you get to contribute as a citizen scientist to the Real-Time Air Quality Map. I was surprised to see just how many PurpleAir monitors there are in New York City, not to mention Brooklyn. I could also see that most outdoor monitors in close proximity to mine were within 5 points of each other.