Entonox Exposure: What the £735,000 Settlement Means for Estates Teams

A group of 121 NHS maternity staff at Basildon University Hospital, 85 of them midwives, has received £735,000 in compensation after breathing in nitrous oxide above the legal limit. An internal hospital report found that some staff were exposed to up to 30 times the workplace exposure limit between 2021 and 2023.

The gas itself isn't the problem. Gas and air (Entonox, a 50:50 mix of nitrous oxide and oxygen) is safe for women to use in labour. The problem is what happens to the gas that is exhaled and leaked into the room, and whether anyone measures it.

Nitrous oxide has a workplace exposure limit of 100 ppm, averaged over eight hours. The only way to know whether a room is inside that limit is to test it, and then to keep testing. This article covers what went wrong, what the engineering evidence says and how often clinics should test, so that other employers can avoid the same outcome.

What happened at Basildon Hospital?

In short, monitoring picked up the breaches, but staff weren't told and the room wasn't fixed for well over a year. The BBC's reporting, based on the hospital's internal report, the independent review and the solicitor who represented most of the claimants, sets out the following sequence:

Summer 2021: air monitoring on the maternity unit showed that workplace exposure limits were being breached.
July 2021: concerns were raised. An independent review by the Good Governance Institute later concluded that the high gas levels should have been treated as a significant risk to health from this point. That didn't happen until October 2022.
2021 to 2023: some staff were exposed to levels up to 30 times the limit, according to the hospital's internal report.
16 months: the time that passed before staff were told about test results that showed breaches.
October 2022: follow-up monitoring, found by chance on a manager's desk, led to staff finding out.
Now: 121 claims have been settled for £735,000, about £6,000 per claimant on average (our calculation). One claim is still to be settled. NHS Resolution reports £172,510 in legal costs so far, a figure it says isn't final.

The Royal College of Midwives (RCM) describes it as the largest group legal action ever brought by midwives in the UK. The trust has apologised and says it should have acted faster. Scavenger equipment is now in use on the unit.

The BBC News report has the full account.

Is gas and air safe to use?

Yes, for women using it for pain relief in labour. The RCM has been clear that the case concerned the conditions midwives were working in, not the gas itself.

The risk is cumulative exposure for staff. A patient uses gas and air for a few hours. A midwife, nurse or dental nurse can spend a whole shift, week after week, in the same room breathing whatever the room holds.

Staff in the case reported fatigue, headaches, brain fog and low B12. HSE says high levels of exposure can cause serious health effects, including neurological problems and anaemia. That is why the limit exists.

What is the workplace exposure limit for nitrous oxide?

The long-term workplace exposure limit (WEL) for nitrous oxide is 100 ppm, or 183 mg/m³, averaged over eight hours. It is listed in EH40/2005 and applies through COSHH. Check the current edition of EH40 for the latest figure.

The limit is an eight-hour average, so the question isn't whether a reading looks high at one moment — it's what a person breathes across the whole shift.

Nitrous oxide has only a faint sweet smell, so staff can't judge exposure by nose. At Basildon, nobody could tell by smell either.

For more on limits across different pollutants, see our guide to workplace exposure limit testing and our nitrous oxide pollutant page.

Where does nitrous oxide in a labour room come from?

There are three main sources. Patients breathe out gas that doesn't stay in the mask or mouthpiece. Connections, tubing, regulators and other equipment can leak. Older pipework can leak too, and the RCM's chief executive pointed to poorly maintained equipment and poor ventilation as what caused leaks to affect midwives at Basildon.

What decides the exposure is how fast that gas is removed. A small labour room with limited fresh air and no scavenging lets it build up through the shift. Add a busy unit and the average rises.

Why isn't ventilation alone enough?

Because air changes per hour describe how quickly a room's air is replaced. They don't tell you where the gas goes or what a person standing in the room breathes.

Two studies give a useful picture. They come from operating rooms rather than maternity units, so they aren't a direct match, but the physics of waste gas is the same.

A study of 35 operating rooms in 10 hospitalsA study of 35 operating rooms in 10 hospitalsmeasured nitrous oxide in the breathing zone of staff across different ventilation and scavenging set-ups:

Ventilation only: levels went above the study's exposure limit in every room.
Natural ventilation plus supplementary pressure ventilation (up to 6 air changes an hour): the limit was exceeded several times.
Pressure and exhaust ventilation (more than 10 air changes an hour) or air conditioning with laminar flow (more than 15), without scavenging: the limit was exceeded twice or less.
Air conditioning or pressure and exhaust ventilation combined with scavenging: levels stayed at or below the limit.
Natural or pressure ventilation combined with scavenging: not enough to keep levels down.

The authors concluded that air conditioning or efficient pressure and exhaust ventilation (above 12 air changes an hour), together with efficient active scavenging, is enough to keep exposure within the limit. Note that the study used a limit of 180 mg/m³, a national figure that is close to the UK limit of 183 mg/m³.

An older engineering study of an operating room and a delivery room mapped how waste gas spreads through a room:

Airflow patterns dominated how gas was distributed. The buoyancy of the gas had very little effect.
Venting waste gas at floor level didn't significantly reduce exposure.
Pockets of high concentration appeared, and where they sat changed with the airflow.
The exhaust grille was the best single place to measure the room average.
In recirculating air conditioning, only the fresh air portion reduces the concentration of waste gas.

This study measured with no staff in the room, so it shows where gas collects rather than what people breathe. That is the point. A fixed reading in one corner can look fine while the midwife's breathing zone, which moves around the bed through a delivery, sits in a different part of the airflow.

"Most trusts will assume a labour room is fine because nobody has complained about a smell and the ventilation was signed off at handover. Neither actually tells you what staff breathe across a long shift. You only find that out by measuring it, and then you have to act on the result."

Adam Taylor, Managing Director, and Chair of the BESA Indoor Air Quality Group

If you want to see how a room's ventilation actually performs, our ventilation validation service measures airflow rates and tests system performance.

What does NHS England guidance say trusts should do?

Guidance issued to trusts by NHS England in 2023 says that:

staff exposure to nitrous oxide should be regularly assessed and monitored
trusts should carry out risk assessments in every area where gas and air is used
trusts should consider specialist ventilation and air purification systems (scavengers) to reduce risks to maternity staff
staff should be trained and made aware of monitoring results

NHS England also says the effectiveness of these measures should be assessed through regular air quality monitoring.

Since the Basildon case, the RCM has worked with HSE to update its guidance on the use of Entonox, and has written to every NHS trust in England about the risks. HSE's guidance on using nitrous oxide safely is worth reading alongside it.

Note the word "regularly" in the NHS England guidance. It is the part that a one-off test doesn't satisfy.

How should nitrous oxide exposure be monitored?

Properly done, nitrous oxide monitoring combines personal monitoring, where staff wear a device through their shift, with area monitoring in the room. Results are compared with the eight-hour limit.

We use both for the reasons the studies above show. Personal monitoring tells you what the person breathes. Area monitoring shows where gas collects and whether the room's ventilation and scavenging are doing their jobs.

A few points that make the data reliable:

Sample across a representative period. HSE notes that exposure in a maternity department varies from day to day and week to week, so one short sample can mislead.
Record what was happening. Note procedure numbers, who was in the room and whether doors were open. A high reading with no context is hard to act on.
Use a competent person. HSE guidance says monitoring should be carried out by someone with the right skills.
Test the whole system. If a result is high, look at ventilation, scavenging and equipment together.

Our nitrous oxide testing service covers all of this, with a report that sets out the results, what they mean and what to do next.

How often should clinics test for nitrous oxide?

EH40 doesn't set a testing interval, so your COSHH risk assessment decides. Our recommendation is a baseline test of every room where nitrous oxide is used, a repeat at least once a year while it stays in routine use, and an extra test after any change.

Re-test when:

a room is refurbished, or a new space starts using gas and air
ventilation is changed, faulty or recommissioned
a scavenging system is installed, serviced or replaced
equipment or pipework is replaced, or a leak is found
the number of procedures or births in a room changes significantly
staff raise concerns about symptoms or the room
a previous result was close to the limit

The testing isn't the whole job. At Basildon, monitoring was used. The 16 months before staff were told, and the further delay before controls were put in, are what turned a set of results into a compensation case. Share every result with the staff who work in the room, and record what was done in response.

A regular programme is also the cheapest evidence an employer can have. A dated series of results, shared with staff and followed by action, shows the risk is being managed.

In short, what should estates and facilities teams do now?

A short check that you can do this week:

List every space where nitrous oxide or Entonox is used. Include maternity, dental, theatres, recovery, day surgery and community clinics.
Check that each risk assessment is current. Look at the date and whether anything has changed since.
Find the last monitoring result for each room. If there isn't one, that room needs a baseline test.
Confirm that results reached staff. If staff haven't seen them, tell them.
Check ventilation and scavenging. Confirm that they perform as designed and that maintenance is up to date. Our HTM maintenance service keeps healthcare ventilation serviced to HTM standards.

If budgets are tight, rank rooms by use. A labour room that handles several births a day sits above a clinic that uses nitrous oxide twice a week, but both deserve adequate monitoring.