Heat-related/Air Quality weather guidelines primarily are based on the Heat Index for Zip Code 20877, as determined by the National Weather Service (using the index that is closest to the start time of any given training or game).  Here is the weblink for that page:
National Weather Service, Zip Code 20877

Heat Index of 90 or less, Code Yellow, Normal guidelines and restrictions
Heat Index of 91-104, Code Orange, Exercise caution; observe players carefully; frequent water breaks; limit time wearing full equipment
Heat Index 105 or higher, Code Red, No outside activities; activities limited to air-conditioned school facilities