The use of gas and air in maternity wards is being suspended amid midwife safety fears.
Multiple NHS Trusts are temporarily stopping the pain relief for pregnant women during labour.
The nitrous oxide levels have been recorded at 50 times the safety limit with concerns raised about staff being exposed to fumes on a regular basis.
The Princess Alexandra Hospital Trust, which operates hospitals in Essex, is among those to axe Entonox gas last month.
They have ordered new machines which break the gas down - though can only get three.
It comes as the Royal College of Midwives has said too many of the buildings housing maternity services are "simply not fit for purpose".
The union claims the issue has arisen from "poor ventilation" in delivery suites and labour wards, which it describes as "the tip of the iceberg".
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Amy Fantis is due to have her baby at Princess Alexandra and said taking away gas and air is "madness".
She told the Times: "It’s not available to everyone and I might not get it. It is a stress that you don’t need.
"I understand they have to keep midwives safe but to just take it away seems madness.
"It is the poor mums who are paying the price."
Joanna Keable, head of midwifery at the Trust, said the issue is solely about the risks of prolonged exposure.
She said gas and air will not bring any risk to expectant mums or their babies.
She said: "Please be reassured that there is no risk to mothers, birthing people, their partners and babies.
"The use of Entonox was temporarily suspended to protect our midwifery and medical teams from prolonged exposure to nitrous oxide in the atmosphere arising from the patient use of gas and air."
Ms Keable added: “There continues to be a range of other pain relief options that our maternity and medical team are discussing with women and pregnant people on an individual basis.
"We have also ordered and are awaiting delivery and installation of permanent extraction/gas and air kits to support us to be able to offer gas and air in all areas of our Maternity Unit as soon as possible."
Ipswich Hospital temporarily suspended its use of the pain relief last year but re-introduced it after new ventilation units were installed.
Dr Suzanne Tyler, the RCM’s Executive Director, Trade Union, said: “Too many of the buildings that house maternity services are simply not fit for purpose. This issue has arisen because of poor ventilation in delivery suites and labour wards – but this is just the tip of the iceberg.
"We know of crumbling walls, ceilings being held up by props and even sewage flowing onto wards.
"During COVID, a lack of decent ventilation and appointments taking place in tiny rooms meant that birth partners were unable to attend scans.
"These aren’t cosmetic issues.
"We need to see proper investment in maternity services, including the bricks and mortar.”
