A chemical distributor has a large shipment of material being sent to a disposal facility. The distributor prepared the material by packaging it into several large containers on two separate trucks. The drivers were instructed to deliver the shipments at the same time. The trucks started the trip in a thunderstorm. One of the trucks had been in a previous incident with an overhead bridge. The damage caused several perforations across the top of the truck. After driving in the rain for nearly an hour, the driver behind the damaged truck noticed fumes escaping from the truck. The contents of the damaged truck had gotten wet. The rear driver called the driver of the damaged truck and informed him of the situation. They both pulled over. Both trucks are carrying boxes of material with blue labels with the number UN 1689.