The Hospital Authority (HA) today (20 August) issued an occupational safety and health alert to remind all public hospitals, in particular Obstetrics & Gynaecology (O&G), operating theatre and dental units, on the possible health hazards resulted from the concentration of nitrous oxide (N2O, also known as “laughing gas” which is used in pain relief) in the ward environment.  The safety alert was issued upon receiving a report from Prince of Wales Hospital (PWH) recently that the concentration of N2O in labour wards has exceeded the occupational exposure limit.

In early June this year, some staff members of the O&G Department of PWH complained about tiredness and headache while at work. They suspected that it was related to the nitrous oxide in the workplace.  While currently there is no legal requirement on routine testing of nitrous oxide concentration in the workplace, as a precautionary measure and to safeguard occupation health and safety, PWH and the Staff Wellbeing Team of HA Head Office conducted an initial environmental assessment in the First Stage Ward 7E and Labour Ward 6EF in mid-June.

A further assessment was conducted with staff personal sampling to measure their level of exposure to nitrous oxide. The sampling by means of wearing monitor badges was carried out in late July. The samples were sent to an accredited laboratory in the United States for analysis. Results indicated that 42 out of 48 samples exceeded the occupational exposure limit of 50 ppm (parts per million of inhaled air) stipulated by the Labour Department, ranging from slightly to 12 times above the limit.

PWH immediately suspended the use of laughing gas in the First Stage Ward and Labour Ward. Alternative pain relieving methods are now in use, such as birth ball, transcutaneous electric nerve stimulation machine, pethidine injection, and epidural anaesthesia.  The hospital has also installed additional exhaust fans and adjusted the ventilation system to improve air flow and fresh air exchange in the areas concerned.

Furthermore, the hospital will also meet the equipment supplier in the next few days to explore measures to enhance facial mask fitting and tubing connection to reduce gas leakage. The use of nitrous oxide would be gradually resumed upon the completion of all improvement measures and satisfactory results from another test conducted by the occupational health and safety team. In the meantime, the airborne concentration of nitrous oxide will be closely monitored to ensure that the standard stipulated by the Labour Department is met.

Regarding follow-up for the staff concerned, PWH will provide preventive medical consultation in the Occupational Medicine / Care Service clinic for O&G staff in need.  Further examinations and follow-up will be arranged if required.  At the moment, medical examination has been provided to five staff members upon their request and there has not been any abnormal report.

According to experts in anaesthesiology, use of laughing gas during labour is safe and effective pain relief method for mothers and their foetuses. Although some studies suggested that prolonged occupational exposure to high levels of nitrous oxide may increase incidence of spontaneous miscarriage of the staff in the workplace, there is limited supporting evidence.  Hence, the risks are considered to be low.

PWH has notified the HA Head Office about the incident via the Advanced Incident Reporting System.

The HA spokesman said the Authority is very concerned about the incident and has notified the Occupational Safety and Health Branch of the Labour Department.  At the same time, the airborne concentration of N2O in all public hospital labour wards is being assessed, while improvement measures are in place to ensure ventilation systems in the concerned wards are functioning effectively to maintain the concentration of N2O within normal levels.

In the safety alert, HA has also reminded health care staff to ensure the proper fitting of N2O breathing masks of expectant mothers and the tubing connection to prevent gas leakage leading to higher airborne concentration of the gas in the ward.  There was no similar report from other hospitals following an initial review of all labour wards.

HA will continue to closely follow up the incident and will establish an Expert Group shortly to look into the incident for recommendations on further follow-up arrangements and appropriate measures.