The fourth open enrollment for health coverage under the Affordable Care Act opened Tuesday, a critical 90 days that the Obama administration hopes will boost participation and stabilize markets roiled by premium increases and insurer withdrawals.

HealthCare.gov and state equivalents began taking applications Tuesday morning from people signing up for individual health coverage and learning about their eligibility for subsidies. This year is especially critical because consumers so far have been sicker and older than expected, which has led to higher-than-anticipated costs.

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