Intended for healthcare professionals


Maternity care: Entonox is withdrawn from some hospitals because of risk to staff from high exposure

BMJ 2023; 380 doi: (Published 27 January 2023) Cite this as: BMJ 2023;380:p212
  1. Jacqui Wise
  1. Kent

The Princess Alexandra Hospital in Harlow, Essex is the latest to suspend the use of Entonox in its maternity unit after air quality tests found levels of nitrous oxide that may put midwives and other healthcare workers at potential risk of harm.

The Health and Safety Executive is already investigating Basildon Hospital, which temporarily stopped using Entonox in December 2022. A BBC report said that nitrous oxide levels found during air sampling were 30 times the legal limit.1

The Royal College of Midwives is considering legal action to support midwives working at Basildon, who have reported a variety of symptoms they attribute to exposure to Entonox.

Entonox, which consists of 50% oxygen and 50% nitrous oxide, has been used for many years as a pain relieving gas mixture. Suspending its use limits the options available to women during childbirth. Alternatives tend to have more side effects and could lead to more interventions during delivery.

Although nitrous oxide is considered a safe option for pain relief during childbirth, prolonged exposure to the gas could lead to vitamin B12 deficiency and anaemia and has been linked to fertility problems. Studies of healthcare workers have investigated whether nitrous oxide increases the risk of miscarriage or produces adverse effects on the fetus, but there is no convincing evidence that this is the case. Midwives are more likely to work long periods in the labour rooms so are potentially at higher risk than doctors who tend to attend intermittently.

Other hospitals that have been affected include Ipswich, which suspended use of Entonox in mid-December, and the William Harvey Hospital in Ashford, which stopped its use for two weeks in November 2022 while new ventilation systems were ordered.

Routine testing at Basildon Hospital in June 2021 showed that nine staff members had been exposed to excess nitrous oxide levels during the course of their shifts. Three showed readings of more than 1000 parts per million of the gas, while a fourth recorded almost 3000, documents seen by the BBC showed. The workplace exposure limit is 100 ppm over an eight hour time weighted average, the HSE said.

The BBC also reported that testing showed high levels more than 16 months before staff at Basildon were told.

An HSE spokesperson said, “We are investigating the levels of nitrous oxide following concerns raised by staff. The higher levels were identified by the Mid and South Essex University Trust during their own air sampling.” The HSE spokesperson added that they were unable to disclose further information because an official investigation had been launched.

A spokesperson for the Royal College of Midwives told The BMJ, “We have been approached by midwife members who have suffered a variety of symptoms they attribute to exposure to Entonox. At Basildon Hospital, investigations have revealed the exposure levels in the maternity departments were over the legal limit. We are investigating all these issues with our lawyers to support our members in potential claims and have reported the issues to the HSE, who are also carrying out their own investigation.”

A spokesperson for Mid and South Essex NHS Foundation Trust said it had followed all of the expert advice and that “significant improvements” had been made. New ventilation systems had been installed, and the trust was continuing to monitor the environment. “We have taken action to resolve the situation and an investigation is under way. Support and advice is being offered to our staff. Mitigations have been put in place, based on clinical recommendations, and we are continuing to keep our staff informed.”


The Control of Substances Hazardous to Health Regulations 2002 requires employers to protect workers from exposure to hazardous substances in the workplace. The regulations require the employer to carry out a risk assessment to establish exposure hazards associated with the substances and processes they are using and to put control measures in place to adequately control exposure.

Good ventilation of maternity units and birthing rooms is vital to protect healthcare workers from excessive exposure to nitrous oxide. Mobile ventilation units or scavenger units can be brought in to help purify the air if an issue is identified. In some cases the extraction system needs to be totally upgraded.

Alex Field, a consultant obstetrician and gynaecologist at the Princess Alexandra Hospital NHS Trust, said, “Following thorough testing of the atmosphere in our maternity unit of nitrous oxide levels arising from the patient use of Entonox, we have temporarily suspended the use of Entonox across the maternity unit to protect our midwifery and medical team. A team of expert engineers tested the system and have ordered local bespoke extraction for the protection of our maternity and medical team. This will allow us to resume the use of Entonox as soon as possible. There are no risks to people attending, staying in, or visiting the hospital.”

Pain relief options

“Gas and air” combinations such as Entonox are a common method of pain relief during labour, but several alternative pain relief options are available. A Royal College of Obstetricians and Gynaecologists spokesperson said, “If a trust does need to temporarily suspend gas and air it is important that this is clearly communicated to women within their antenatal care.”

One alternative is a pethidine injection into the thigh or buttock. However, the effects last between two and four hours so this is not recommended if the second stage of labour is close. Also, if it is given too close to the time of delivery it may affect the baby’s breathing and can interfere with the baby’s first feed.

Another alternative is remifentanil, which works quickly and wears off after a few minutes so can be used up until the baby is born. However, like pethidine, remifentanil can affect the baby’s breathing, although this usually wears off quickly.

An epidural is another option, but this can prolong the second stage of labour. And because the person giving birth may no longer feel the contractions, instrumental delivery is more likely.

Giles Thorpe, chief nurse at East Suffolk and North Essex NHS Foundation Trust, told The BMJ, “We completely understand the concerns of women and pregnant people who would like a hospital birth with the option of using gas and air.” But he added, “Until there is a safe and effective solution, we are unable to reintroduce gas and air to ensure we keep our maternity team safe.”