US Marine Corps Heat Stress Guidelines

The US Marine Corps developed the WBGT heat index in the 1950's in response to heat-related casualties during training. By 1998, the Marine system had evolved into a fairly complex structure. WBGT measurements are taken hourly from 7 am to 8 pm daily from the 3rd Monday in April until the 3rd Monday in October, using non-electronic instruments. When WBGT reaches 84°F, measurement frequency is increased to every 30 minutes. WBGT measurements are taken from multiple stations around the island. Each station runs its own heat risk flags. The current flag system is as follows:

WBGT 90°F and above = "Black Flag" - All outdoor training requiring exercise stops.

WBGT 88-89.9°F = "Red Flag"

WBGT 85-87.9°F = "Yellow Flag"

WBGT 75-84.9°F = "Green Flag"

In addition, there is a "cool room" where personnel experiencing heat illness are kept. When this "cool room" is filled, there is "Administrative Black Flag" status, to prevent generation of further need for "cool room" treatment. Activities permitted during each flag condition vary according to station. Almost all outdoor training requiring vigorous effort is stopped during Black Flag conditions. Limitation of activity during other Flag conditions is determined by the type of activity and the acclimatization and conditioning status of the recruits involved.