A school kitchen was closed after reports of a carbon monoxide leak.
The detector went off at Leavesden Primary School last Tuesday.
Investigators and health and safety executives were called out to investigate on Wednesday.
They found "safe" levels of carbon monoxide and said there was no risk to life, according to Caterlink, which runs the kitchen at the school.
It has since reopened.
Two members of staff, who have since returned to work, reported feeling sick.
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