If the left side of the NFPA label is rated 4 in blue, what does that indicate?

The left side of the NFPA (National Fire Protection Association) label indicates the health hazard rating of a substance. A rating of 4 in blue signifies a severe health risk. This means that even short exposure to the substance can lead to serious injury or potentially death. The NFPA labeling system uses a scale from 0 to 4, where 4 represents the highest level of hazard, indicating an immediate threat to life and health. In a hazardous materials context, it is crucial for responders and workers to recognize and understand these ratings for their safety and to take the necessary precautions when handling such materials.

The other options do not align with the meaning of a 4 rating in health hazards in the NFPA system. Non-toxic substances would not be rated 4, nor would low flammability or environmental safety relate directly to the health hazard assessment indicated by that specific rating.