None of Congress' proposals to rein in surprise medical bills address ambulances — which are expensive and often aren't covered by insurance.

Why it matters: More than half of ambulance rides, and two-thirds of air ambulance transports, aren't covered by private insurance. Patients are often billed more than $10,000 for what insurance won't cover.

What we're watching: Energy and Commerce Committee Chairman Frank Pallone has said that the committee plans to address air ambulances in its final bill. But it's less clear what will happen with ground ambulances.

What they're saying: "Ground ambulances are arguably the screwiest market of any that comes up in this context," said Brooking's Loren Adler.

"Neither side has much incentive to contract because the insurer knows the ambulance has to pick up anyone who calls 911 and the ambulance doesn’t want to take less money from the insurer than they could get balance billing people," he added.

The other side: Air ambulances say that the government reimburses below the cost of the service. That means that they have to charge privately insured patients higher rates, but insurers often refuse to cover their services.

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