Construction is a very dangerous line of work. Construction workers must do manual labor with heavy machinery in less-than-ideal weather conditions. As a result, the construction industry sees an abnormally high rate of workplace injuries.

The Occupational Safety and Health Administration (OSHA) has identified four injuries that are particularly dangerous for construction workers. These four types of accidents claimed the lives of 631 workers in the United States in 2016. The administration even has a nickname for these deadly injuries: The Fatal Four.

1. Falling

Construction workers often have to scale ladders, scaffolding, cranes and roofs. These heights can pose significant risk of a fall. According to OSHA’s data, falls contributed to 384 deaths in 2016 and were the leading cause of construction-related fatalities. Sometimes, falls are caused by unsafe working conditions.

2. Being struck by an object

Ninety-three construction workers were struck by objects and killed, per OSHA’s report. Though most construction sites require hardhats and other safety equipment, employers are not always scrupulous in enforcing these standards. Employees can incur brain and spinal injuries from being struck on the head by a falling object—even when wearing safety equipment.

3. Being electrocuted

The third culprit in the Fatal Four is electrocution. Electrocutions can be caused by faulty wiring, electrical tools coming into contact with wire, frayed cords or downed powerlines. Though most electric shocks are mild, shocks that are severe enough can cause death. In 2016, no fewer than 82 people were killed via construction-related electrocutions.

4. Being caught between an object

Rounding out the four most common fatalities to construction workers is being caught between something. When a construction worker is caught between a piece of heavy machinery, pinned against a wall or crushed by a collapsing structure, the consequences can be dire. Seventy-two people were killed in these incidents. Sometimes, these fatalities were the result of unsafe workplace conditions and could have been prevented.