Which type of physical hazard can self-ignite upon contact with air?

The correct answer is pyrophoric. Pyrophoric materials are substances that can ignite spontaneously in air at or below 130°F (54°C). This self-ignition occurs because these materials are often very reactive and can rapidly oxidize upon exposure to oxygen in the atmosphere.

Understanding this characteristic is crucial for safety in handling such materials, as pyrophoric substances can pose significant risks if not managed properly. Proper storage and handling protocols must be in place to mitigate the risks associated with their potential to ignite without an external ignition source.

In contrast, oxidizers, flammable, and combustible materials have different ignition characteristics. While oxidizers can enhance the combustion of other materials, they do not spontaneously ignite upon exposure to air. Flammable materials require a specific concentration of vapor in the air along with an ignition source to ignite, whereas combustible materials are those that can catch fire at higher temperatures compared to flammable materials but also do not spontaneously ignite upon contact with air.