Conflict between environmental and health and safety legislation led to two delivery workers being overcome by toxic fumes from rotting animal waste at a Scottish rendering plant.

Forfar Sheriff Court heard on 25 June that Charles Anderson and Richard Dow were poisoned by potentially lethal hydrogen sulphide on 18 July 2007, as they delivered freshly slaughtered cattle from an abattoir to a waste pit for incineration.

The men collapsed and lay unconscious on the floor in the enclosed waste intake area of the plant before being rescued and taken to hospital, where Mr Anderson was in a critical condition for some time. They have both since recovered.

Investigating HSE inspector John Radcliffe told SHP that Sacone Environmental, the operator of the plant at Brechin, Angus, had been trying to comply with orders from the Scottish Environment Protection Agency (SEPA) but, in doing so, created avoidable health and safety risks.

"The environmental legislation kicked in because the process is very smelly," explained the inspector. "SEPA had made the firm create a virtual confined space by specifying that doors should be put on the receiving area to stop odours emanating into the general atmosphere. But by keeping the smells in, this also kept the gas in the breathing zone of workers."

The company had used the fact that it had to comply with environmental legislation as part of its mitigation. It said it had also provided monitoring alarms and ventilation since the incident.

But inspector Radcliffe said: "Clearly, the company needs to comply with health and safety, as well as environmental, legislation. There can be a conflict between SEPA and the HSE, but it is possible to comply with both bodies by installing carbon pack filters to take the smell away before it goes into the atmosphere. The company had been somewhat preoccupied by environmental considerations and had overlooked the health and safety aspects."

The inspector concluded: "This accident was entirely foreseeable and preventable, as it was foreseeable that hydrogen sulphide could accumulate in this plant. Such areas should be well ventilated, and gas-monitoring equipment – designed to provide an alert in the event of a gas build-up – should have been provided. There was no quick means of escape from this area of the plant, nor was protective respiratory equipment provided."

Sacone Environmental pleaded guilty to breaching s3(1) of the HSWA 1974 by failing to ensure the safety of non-employees and was fined £12,000. No costs are awarded in Scottish courts.