The correct answer to this open question is the following.
The OSHA standard mandates that any time a worker is at a height of six feet or more, the worker is at risk and needs to be protected. Most fatalities, however, happen when workers fall from falling hazard.
OSHA stands for Occupational Safety and Health Act. It was enacted in 1970. Its mission is to "assure safe working conditions for workers."
In the construction business, falls are the number one cause of fatalities. That is why OSHA includes a section called "Subpart M" that lays out the requirements and criteria for the protection of the workers and to avoid falls in the workplace.