It's hard to predict when or where hazmat accidents will occur, but when they do, environmental reporters are often among the first journalists sent to the scene. For journalists, as for other "first responders," preparation makes for effectiveness.
What's in that derailed railcar or overturned tanker truck? The answer is often visible on a placard affixed to the vehicle (we suggest binoculars). The federally-required hazmat placard often includes a "UN number" specifying what's in the railcar. You can look up what these numbers stand for in the "hazmat table" published in the Code of Federal Regulations, among other places.
It will do little good if you have to search for it on your way out the door — better to have a paper copy in your backpack or an electronic copy in your laptop.
- "Hazardous Materials Table,"  49 CFR 172.101.
Another good reason to keep a copy is that there have been from time to time (especially after 9/11) proposals to make this information secret for homeland security reasons. Firefighters have been among those opposing such secrecy.