Staff at a maternity unit were exposed to almost 30 times the legal workplace exposure limit for nitrous oxide, documents have shown.
Testing at Basildon Hospital revealed the levels more than 16 months before colleagues were informed.
The Royal College of Midwives said its members there were considering legal action.
Routine testing of the maternity suite in June 2021 revealed nine staff members had been exposed to excess nitrous oxide levels during the course of their shifts.
Three had readings of more than 1,000 parts per million (ppm) of the gas, while a fourth recorded almost 3,000. The Workplace Exposure Limit is set at 100ppm.
Trust management apologised after failing to notify staff at the unit until October 2022.
A briefing seen by the BBC stated the issue was logged on the risk register, but "there has not been proper oversight of the problem and staff have not been informed".
One person familiar with the situation, who did not want to be identified, said: "We had an email sent out that said 'emergency maternity staff briefing' and there was a Teams meeting.
"The Teams meeting was very, very difficult to listen to. It was very emotive. People were angry understandably, but I feel like the executive who were on the call didn't handle it very well."
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Source: BBC News, 16 January 2023