Manufacturers of the mushroom caps agree that those caps were designed to provide scratch protection only and were never intended to prevent impalement, even at grade.
OSHA goes on to say, “Considering the serious nature of the hazard, the standard mushroom style plastic rebar caps should not be used for protection against impalement. Protective devices (covers or wooden troughs) capable of withstanding at least 250 pounds dropped from a height of ten feet should be used. OSHA doesn’t approve products, but there are steel reinforced covers and plastic and wooden troughs available that provide the needed protection.”