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Safety Triangle Analysis

The safety triangle, also known as Heinrich’s triangle, Bird’s triangle, accident triangle, or safety pyramid, is a theory of industrial accident prevention. The Heinrich triangle seeks to develop links between fatal accidents, minor accidents, and near misses. It suggests that reducing the number of minor accidents will lead to a fall in fatal accidents. 

Herbert William Heinrich first proposed the safety pyramid in 1931, hence the term, Heinrich accident triangle, derived from his last name. Moreover, other writers like Frank E. Bird, thus also known as the “Bird Safety Triangle” have since expanded upon the theory.  A triangle or pyramid often shows this relationship pictorially. Analysts have described it as a cornerstone of 20th-century workplace health and safety philosophy. However, recently, it has come under criticism over the scores designated to each accident category and for focusing on reducing minor injuries.

Heinrich Safety Pyramid Theory

The Heinrich safety pyramid shows the link between the volume of accidents causing significant, minor, or no injuries. The Heinrich triangle was first proposed in 1931 by Herbert William Heinrich in his book Industrial Accident Prevention: A Scientific Approach. He was a workplace health and safety leader and worked in the insurance sector. So, he had an interest in reducing the number of serious industrial accidents.

Since fewer accidents would mean his company would have to pay fewer claims, he commenced a study of more than 75,000 accident reports. He compiled this data from his company’s files and records held by individual industry sites. From this data, he deduced a relationship between one major injury accident, 29 minor injury accidents, and 300 injury-free accidents. He established a direct connection between minor and major accidents.

By reducing the number of minor accidents, factories would also see a correlating fall in the number of significant casualties. Various health and safety programs have used this safety triangle for about 80 years. According to the Heinrich Safety Pyramid theory, human error causes 88 percent of all accidents.

Revision of the Heinrich Safety Pyramid

Frank E. Bird later contributed to the Heinrich Safety Pyramid theory in 1966. He based his findings on 1.7 million accident reports from almost 300 companies. So his updated triangle, known as “the Bird’s Triangle,” showed a relationship of one serious injury accident to 10 minor injury accidents, 30 damage-causing accidents, and 600 near misses. Therefore, he established a link between the number of near misses and the number of significant casualties. Later, he claimed that human intervention could predict and prevent most accidents.

Moreover, a 1974 study titled “The Human Element in Systems Safety” by A. D. Swain supported Bird’s triangle theory. Hence, they both expanded on this study in their book Practical Loss Control Leadership in 1985.

Components of a Safety Triangle


Heinrich's Triangle Theory
Heinrich’s Triangle Theory

The Safety Triangle or Heinrich Triangle consists of five sections. As you go up the Heinrich Triangle, each part suggests a further unsafe act or occurrence. One can also learn about Heinrich Safety Pyramid Theory in detail by taking OSHA 30-Hour Construction Outreach Training.

What does the Accident Triangle represent?

The Safety Triangle, also known as the Accident Triangle, illustrates the relationship between accident severity and accident frequency. Moreover, it envisions the relations and normal distributions between fatal or severe accidents, minor accidents and near misses.

Starting with the most severe consequences, details are below on all aspects of the safety triangle:


Fatality is the most critical aspect of the safety triangle. First, it demonstrates the occurrence of death by accident.

Lost Time & Severe Injury:

The definition of workplace injury covers both accidents resulting in disabilities as well as those not resulting in injuries. These injuries may include:

  • An incident causing severe eye damage, requiring a trip to the Doctor
  • An accident resulting in broken bones
  • Injuries requiring hospitalization
  • An accident resulting in unconsciousness for the involved workers
  • Accidents requiring treatment by a healthcare professional
  •  Illnesses resulting in loss of mobility

Minor Injuries:

Any injury that does not require hospitalization or a visit to a Doctor. For instance, a wound can be treated immediately on-site, like cuts, minor wounds, or scratches. So they are non-life-threatening injuries.

Near Misses

It is an incident in which a worker narrowly avoids getting injured.

Unsafe Acts:

Any instance of a breach of safety protocols is an unsafe act. Such instances will raise the probability of accidents at the workplace.


The most effective way to prevent injuries and accidents in any workplace is through completing OSHA 30-hour online training courses since these courses include all essential site safety topics that train workers on recognizing, controlling, and preventing potential site hazards.

Criticism on the Heinrich Safety Pyramid Theory

The Heinrich Safety Pyramid theory profoundly impacted the health and safety culture in the 20th century. However, analysts have criticized some aspects of this Safety Triangle theory recently. Some of this criticism relates to the use of exact figures. A 2010 report about the oil and gas industry showed that the original values held when applied to an extensive dataset and a wide range of activities. A 1991 study suggested that the relationship was markedly different in confined spaces. There were 1.2 minor injuries for each severe injury or death. A broad survey of UK accident data in the mid-1990s showed a relationship between one end to 207 major injuries, 1,402 injuries causing three or more days lost time, and 2,754 minor injuries. Unfortunately, Heinrich’s original record has since been lost. So it is impossible to prove his accident figures.