Why spills happen

Identifying why spills happen can help those of us who use, handle, or transport oil focus our oil spill prevention efforts where they will do the most good.

When a spill to water — or substantial threat of spill to water — occurs, we investigate the immediate cause and contributing factors to support prevention efforts. This causal information also may inform enforcement decisions. We classify the immediate cause and contributing factors using a data dictionary developed by the Pacific States/British Columbia Oil Spill Task Force. We began using the data dictionary in 1999.


  • Immediate cause: What happened immediately prior to the incident — a triggering factor. Only one immediate cause may be associated with an incident.
  • Contributing factors: Underlying conditions that set the stage for or worsen the incident. Multiple contributing factors may be associated with an incident.

We use three levels of investigation effort, based on our evaluation of each case. Immediate cause and contributing factors from the highest two tiers of investigation effort — which include internal peer-review and the opportunity for input from the responsible parties — we consider to be high-confidence data. Those high-confidence data are presented on the first tab in the below interactive graphing tool.

The second and third tabs provide a “Pareto view” of the distribution of immediate cause and contributing factors from the entire data set. The Pareto analysis, sometimes called the “80/20 rule” or “law of the vital few,” shows those categories of cause that occur most frequently in our dataset. The idea is that entities that handle or use oil may best advance their oil-spill-prevention goals by first focusing on the few categories of cause that cumulatively result in most spill-related incidents.