By tightening requirements for charity care, hospital executives say, they hope to encourage eligible people to obtain low-cost insurance through the subsidized private plans now available under the law.

“Do we allow our charity care programs to kick in if people are unwilling to sign up?” said Nancy M. Schlichting, chief executive of the Henry Ford Health System in Detroit. “Our inclination is to say we will not, because it just seems that that defeats the purpose of what the Affordable Care Act has put in place.”

But advocates for the uninsured point out that many Americans avoided obtaining coverage in the inaugural enrollment period of the Affordable Care Act this year because they found the plans too expensive, even with subsidies. Many uninsured people also remain unaware of the new insurance options, And immigrants who are in the country illegally are not even eligible to apply.

“Certainly we want to encourage people who have new access to affordable coverage to take advantage of it,” said Sidney D. Watson, a professor at St. Louis University’s Center for Health Law Studies. “But I think we’re all going to have to do a lot to get that message out, and there will always be people who won’t have the option.”

Beverly Jones, 51, of St. Louis, who has lupus, is the type of person targeted by Barnes-Jewish Hospital’s new policy. Ms. Jones, who already owes Barnes-Jewish thousands of dollars for emergency room treatment and other visits, said the hospital’s new co-payments for the uninsured would “throw my budget into a tailspin” on her annual income of $13,400, which comes mostly from disability checks.