A growing number of people are turning to health-care ministries to cover their medical expenses instead of buying traditional insurance, a trend that could challenge the stability of the Affordable Care Act.

The ministries, which operate outside the insurance system and aren’t regulated by states, provide a health-care cost-sharing arrangement among people with similarly held beliefs. Their membership growth has been spurred by an Affordable Care Act provision allowing participants in eligible ministries to avoid fines for not buying insurance.

Ministry officials estimate they have about 500,000 members nationwide, more than double the roughly 200,000 members before the law was enacted in 2010.

The membership growth was largely unanticipated by ministry officials when the groups obtained an exception to the law. Only ministries in continuous operation since at least Dec. 31, 1999 are exempt from the ACA. The carve-out was intended to satisfy what at the time were relatively small religious groups that argued that their nonparticipation was a matter of religious freedom.

But now, some insurance commissioners are concerned that the ministries could put consumers at risk if bills aren’t paid. The ministries aren’t overseen by state commissioners, which generally guard against unfair practices and ensure solvency.

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