Both of these training designations fall under 29 CFR 1910.120 (e). The main difference between the two is that one allows a person to work at a contaminated site for a prolonged period of time. The other one does not. The Hazardous Waste Operations and Emergency Response (HAZWOPER) 24 is mainly for short work durations at
HAZWOPER designated sites. Because of the shorter work period, it is unlikely that a person will work around a hazardous substance(s) at or above its Permissible Exposure Limit (PEL). The PELs can be found in the Z tables of the Occupational Safety and Health Administration (OSHA) standard 1910.1000
The
HAZWOPER 40 course is designed to prepare individuals to work at contaminated sites while being exposed to hazardous substances at or above PELs. The training will go into a great deal of detail in wearing a Level A suit along with wearing either an inline respirator or a Self Contained Breathing Apparatus (SCBA). Donning and doffing Level A suit equipment usually takes 2 people. In contrast, a person trained to the
HAZWOPER 24 level will likely wear Level C equipment when conducting work at a site. This equipment is usually donned and doffed by the person wearing it with no help from anyone else.
After the initial 40 or 24 hour training is completed, each individual must go through site specific training by a qualified supervisor. The requirement is 3 days for the 40 hour course and 1 day for the 24 hour course. Hands-on training of Personal Protective Equipment (PPE) and other equipment is required for both - 40 hour and 24 hour. 
