CNN answers six key questions on the derailment of the train carrying hazardous material earlier in February:

Why are hazardous materials transported by train?

This is in large part because railroads are considered the safest mode of transportation to carry large amounts of hazardous materials, including chemicals, for long distances across the country, according to the Federal Railroad Administration. America’s freight trains moved 2.2 million carloads of chemicals in 2021, according to the Association of American Railroads.

How often does this kind of incident happen?

Federal Rail Administration data provided to CNN showed 149 incidents where hazardous materials were released from moving trains over the past decade. It’s important to note the federal data is being self-reported by train companies, making it tough to verify all the information is accurate.

How do officials measure the damage and impact?

Ohio Environmental Protection Agency officials have been conducting air, soil and water tests since the controlled explosion of the chemicals inside the rail cars.

The EPA said Tuesday there were chemicals spilled into the local waterways that lead to the Ohio River, but that much of it was contained. An initial plume of chemicals that was spilled into the waterway had made it to the Ohio River, but officials said they exist in very low concentration, and they are working with water facilities on enhanced filtration so they are not passed onto water customers.

That said, EPA and Ohio state officials urged residents in East Palestine to continue to drink bottled water for now.

If animals are dying, what does that mean for humans?

The chemical spill took a severe toll on local aquatic life: About 3,500 fish ranging across 12 species died from the water contamination washing down streams and rivers. Anecdotal reports of pets and chickens dying have not yet been confirmed by officials. Mary Mertz, the director of Ohio’s Department of Natural Resources, said Tuesday there’s no evidence that non-aquatic species have been affected by the spill.

What goes into cleaning up?

Ohio EPA and state officials have done several different things to try to contain pollution from the chemical spill.

What are the possible long-term impacts?

In addition to the chemicals officials say should break down with aeration and water treatment, environmental officials also need to test for PFAS – a long-lasting and potentially more worrying class of chemicals used to put out chemical fires. PFAS is typically found on non-stick pans and in some firefighting foams.

Read more details here.