More than 4 million Americans get subsidies on the federal exchange used by up to 36 states. If the subsidies ultimately are struck down, more people could qualify for hardship exemptions based on their inability to afford coverage.

The exemptions are worrying insurers. The penalties were intended as a cudgel to increase the number of people signing up, thereby maximizing the pool of insured. Insurers are concerned that the exemptions could make it easier for younger, healthier people to forgo coverage, leaving the pools overly filled with old people or those with health problems. That, in turn, could cause premiums to rise.

Patrick Getzen, vice president and chief actuary at Blue Cross and Blue Shield of North Carolina, said he saw more “older and sicker people” enrolled in 2014 than projected. He attributed some of that to the weakened mandate. “With a stronger penalty and less broad exemptions, that would be better for the risk pool.”