These human factors bulletins focus on how workers interact with their work environments. They are based on accident investigations that examine all the workplace factors that influence the decisions and actions of the workers involved in an accident. These factors help to identify the causes of an accident. Identifying these causes can help to prevent similar workplace accidents.

Assessing risk? Consider human limitations

Risk assessments evaluate the hazards of a work task that put workers at risk of injury or death. This bulletin looks at how a risk assessment of a work task needs to consider the limitations of the individuals doing the task.

What happened?

A worker was preparing to mix chemicals in a large in-floor vat in order to make lacquer. After removing the bungs from a steel barrel of toluene, the worker lowered the barrel onto its side and placed it on the lip of the in-floor mixing vat. As the toluene was being poured into the vat, a static discharge was produced that caused the toluene vapour to ignite. This in turn caused the toluene liquid to explode, fatally burning the worker.

From a human factors perspective, why did it happen?

At the time of the incident, there were two grounding systems in place that could have prevented the static discharge. One was a manual grounding system (portable grounding clamps), and the other a passive grounding system. The portable grounding clamps were available at each in-floor vat for the worker to apply when transferring chemicals. The passive grounding system consisted of a steel floor plate surrounding the vats. The plate was attached to a ground wire. The passive ground system was reportedly put in place approximately four years ago when a similar incident occurred. The purpose of this system was not to replace the manual ground, but to add another line of defense to the system. However, in order for this passive ground system to be an effective defense, there needs to be direct metal to metal contact.

The evidence indicates that the worker had not manually attached the portable grounding clamps to the vat before transferring the chemical. Why did this happen?

The process for transferring chemicals from one container to another was evaluated to determine if there was any safeguard in place to remind a worker to apply the manual grounding system. Relying on memory for critical functions is one of the least effective ways to maintain safety. In this incident, the worker may have simply had a lapse in memory. There was nothing in the worker's environment to warn him that he had omitted doing this crucial step. This meant that his only defense now was the passive grounding system. The subsequent explosion showed that this secondary measure was ineffective. Why did it fail?

For the passive grounding system to work, the barrels need to touch the steel plate (i.e., metal to metal). On the day of the incident, investigators saw flattened cardboard boxes at the site. Interviews with co-workers disclosed that the cardboard was used sometimes to help workers manipulate the barrels. Cardboard was placed under the barrels as they were lowered to their sides to provide traction with the floor. The worker was over 50 years old and weighed less than 200 lbs. The 45 gallon barrels he was handling could run upwards of 300 to 400 lbs. (Some of the 45 gallon barrels observed on site after the incident revealed labels suggesting a weight of 440 lbs.). It is not known if the worker used cardboard to help him lower the barrel on the day of the incident. But if it was used, it would have rendered the passive grounding system ineffective.

Understanding human limitations helps avoid workplace accidents

Human memory is fallible and cannot be relied on as an effective safety measure. There was nothing in the workplace to remind the worker to attach the grounding wires if he missed this crucial step. Although there was a secondary defense measure (the passive grounding system), there were no cues to indicate to the worker whether this system was implemented and/or effective. The outcome of these safety measures not being applied or being ineffective was catastrophic.

The daily task of pouring chemicals into the large in-floor vats involved very heavy manual handling. Without formal re-design or modification to assist workers in performing heavy manual handling activities, it is likely that they will make modifications to the tasks themselves in order to make them more manageable. In this instance, it is possible that these modifications may have rendered the secondary defense system ineffective.

Developing or evaluating a work task using a risk assessment should include the human limitations of individuals performing the work. In this particular task, human memory and physical demands of the job were limitations that needed to be considered in the risk assessment.