from 2.0 to 3.9 pCi/L
Each short-term test is a snapshot of conditions during the 3-7 days of testing (for a short-term test), so in this range, we recommend you test again. You may want to retest during another season of the year.
If the follow-up results are still in this range there is little short-term risk, but the decision of whether to pursue radon mitigation is a personal one, balancing costs and risks. While the United States EPA recommends action at or above the 4.0 pCi/L level, they also state:
"A radon level below 4 pCi/L still poses a risk. Consider fixing when the radon level is between 2 and 4 pCi/L."
(from Home Buyer's and Seller's Guide to Radon by US EPA)
The World Health Organization has established an action level of 2.7 pCi/L.
If you change the way you use your home, test again.