Final answer:
In the ERG, a "P" following the guide number indicates that the material is toxic and may cause serious health effects.
Step-by-step explanation:
In the context of the Emergency Response Guide (ERG), a "P" following the guide number indicates that the material is toxic and may cause serious health effects. Specifically, the letter "P" stands for poison. It is used to provide emergency responders with information about the potential hazards associated with the substance.
For example, if a substance is labeled with a guide number of 126P, it means that the material is poisonous and can cause adverse health effects if not handled properly. This information helps emergency responders take appropriate actions to protect themselves and others.