After smoking, radon is the top cause of lung cancer in this country—it’s responsible for some 21,000 deaths a year, according to the EPA. Overexposure is symptom-free, and once you’re exposed, there’s no treatment. Detecting radon and fixing the problem are far easier and less expensive than dealing with lead.

Radon is measured in picocuries per liter (pCi/L); 1.3 is considered the national average indoor level. Although 4 pCi/L is the recommended EPA action level, the agency also suggests that you consider remediation at a level between 2 and 4 pCi/L.

We tested seven short-term (2- to 7-day) radon kits, three long-term (90-day) kits, and a digital-readout meter using experts at two labs. Here’s what we found:

Long-term radon kits are more accurate. The radon level can vary significantly from day to day. Sampling the level for 90 days or more gives you a more accurate idea of the average radon level in your home. If you need fast results, only one short-term radon kit, the RTCA charcoal canister, was accurate enough for us to recommend. But you should still confirm its results with a long-term test. The $120 Safety Siren Pro digital meter can be used for short- or long-term testing.

Some radon kits underreported. The Accustar CLS 100i and the Kidde short-term radon kits underreported radon levels by almost 40 percent.