The workers were using a petrol saw to cut out an existing concrete floor at a fish factory in Hull. In order to protect the factory's surfaces from dust, they created a sealed enclosure from timber and polythene.
The space was not ventilated and in October 2015 there was a build-up of carbon monoxide from the saw, which left one worker hospitalised.
The Health and Safety Executive's (HSE) investigation found Westland had not planned the work and failed to consider the dangers of an un-ventilated tent.
It said the company should have prevented the workers' exposure to harmful dust with a suppression system, local exhaust ventilation and appropriate respiratory protective equipment.
Westland was fined £16,000 plus costs of £847 at Hull Magistrates' Court.
In its Health and Safety Policy Statement of Intent posted online, the company says it works to prevent accidents, injuries and damage to health by "providing safe working environments that are without risks to health", "safeguarding employees and others from foreseeable hazards connected with work activities", and "ensuring that, when new substances, equipment, processes or premises are introduced, adequate guidance, instruction, training and supervision are provided for safe methods of work to be developed".
After the hearing, HSE inspector Jennifer Elsegood said: "Petrol-driven saws should not be used in a confined space because of the risk of carbon monoxide exposure. Carbon monoxide is extremely dangerous; it has no smell and workers can be overcome by the fumes before they realise they have been affected, making it extremely dangerous. This is why it is known as the 'silent killer'."
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