This past winter, I played the role of reluctant shut-in. I was recovering from a health calamity which forced me to spend my days working from a sofa in my den. I had never spent so much time inside my home—a 131-year-old, two-and-a-half-bedroom, fourth-floor apartment in Brooklyn’s Cobble Hill Historic District. I live here with my three children, and we consider ourselves lucky to have nine windows on three exposures. I feel less lucky about the constant hum of the six-lane Brooklyn Queens Expressway one block away, or the hiss of the old-fashioned steam heat system.

I also suffer from a perennial sinus infection that usually shows up during February or March, then blossoms into a full upper respiratory infection before the grand finale: situational asthma that requires two different inhalers; prednisone; and during one particularly bad bout, a nebulizer at urgent care. I wasn’t sure when my yearly scourge would make its appearance, but I was certain it would be soon.

A public relations rep from a company called Airthinx offered me a review unit of its consumer air-quality monitor, which measures indoor air pollution. (I'm lucky to work in technology journalism, where I get to borrow expensive equipment for testing purposes.) I was curious. I never really considered the quality of my air before, and as long as my carbon monoxide monitor and smoke detector didn’t go off, I assumed all was well.

A week later, I opened what looked like a See’s Candy box to find the small, white, plastic Airthinx. I plugged it in and watched a thin beam of red light glow across the top. It wasn't an alert telling me to set up a Wi-Fi link—the device costs $700 and requires a monthly $30 subscription that, in part, goes toward always-on 3G connectivity. That red light was a warning: I had Poor Air.

The Airthinx uses nine built-in sensors to measure key air quality indicators. There are the obvious ones like temperature, humidity, and carbon dioxide; ones that don't seem to matter like air pressure and organic compounds; and the ones you don't think about like formaldehyde and particulate matter. Particulate matter is a catch-all term. It can be dust, smoke, soot, viruses, fungi, or bacteria, among other things. Our nose hairs capture the larger particles measuring down to 10 micrometers in diameter, and instead of breathing those particles into our lungs, we swallow or expel them. It’s the tinier pieces—2.5 micrometers or smaller—that can lodge deep in the lungs. And if the particles are toxic, like bits of dust from asbestos, they can be deadly. The World Health Organization estimates 5 percent of lung cancer deaths can be attributed to particulate matter.

I opened the Airthinx app to see that my Indoor Air Quality score was 72 out of 100. Even though zero is the worst possible score and 100 indicates pristine conditions, 72 ranks as poor? Why? Well, the particulate matter in my apartment measured almost twice the outdoor average for my area of Brooklyn (maybe because I was so close to the BQE?) and the "volatile organic compounds" measured more than three times what the WHO recognizes as "good." My place was also too hot and too dry, according to the Airthinx.

Vacuuming with my small canister vac only made my PM and VOC levels spike. Plugging in my tiny humidifier didn't boost the humidity but somehow increased the PM score. I couldn’t get good air, and it was obvious that I was trying the wrong remedies. Meanwhile, the Airthinx silently watched me with its judgy red light.