Health and Safety
Executive / Commission
Regulatory Impact Assessments: Chemicals
1. Hydrogen selenide is included in a Commission Directive on Indicative Occupational Exposure Limit values (IOELVs). Members states are required to have set a national occupational exposure limit for hydrogen selenide that takes into account the IOELV by 31 December 2001. The IOELV for hydrogen selenide is 0.02 ppm (8-hour TWA) and 0.05 ppm (15 minute reference period).
2. WATCH considered the risk assessment on hydrogen selenide in January 1998 and concluded that there was insufficient data to identify a safe level of exposure at which there would be no risk of eye and respiratory tract inflammation. Therefore, the existing Occupational Exposure Standard of 0.05 ppm 8-hour TWA was withdrawn in March 1999.
3. Because no safe exposure limit for hydrogen selenide could be identified, ACTS considered setting a MEL. However, ACTS decided that a MEL was not appropriate for hydrogen selenide as current occupational hygiene data shows that very few people are exposed to the gas and that the potential levels of exposure are likely to be negligible. A Chemical Hazard Alert Notice (CHAN) providing interim advice on the handling of hydrogen selenide was issued in January 1999.
4. However, there is now a need to comply with the IOELV Directive and since the criteria for an OES are not met a MEL is the only option.
6. The critical health effect is irritation to the respiratory tract. There are no reliable data from animal or human studies concerning the dose-response characteristics for this effect. Furthermore, there are no animal or human data for sensitising, genotoxic, carcinogenic or reproductive effects of hydrogen selenide.
7. Early reports of uncertain reliability indicate that employees couldn't work without respiratory protection at concentrations above 1.5 ppm. Symptoms of nausea, vomiting, a metallic taste in the mouth, dizziness and extreme lassitude have been reported within one month of exposure to the gas at exposure levels of between 0.3 - 1.5 ppm. However, it is not possible from the data available to establish a NOAEL for respiratory irritation.
8. Occupational exposure to hydrogen selenide in Great Britain (GB) may occur during its handling for distribution by three companies and during its use by six companies as a dopant gas in the preparation of semiconductor materials. There is no occupational exposure data available from the suppliers of the gas or the main users. The EASE model predicts that occupational exposures during cylinder changing by the users and in the event of any potential leakage into the glove box during gallium arsenide wafer production are 0.002-0.010 ppm (15-minute reference period) and 0-0.0003 ppm 8-hour TWA respectively.
9. Hydrogen selenide may be produced as a by-product and liberated during processes where selenium compounds may be present for example, during the smelting of alloys, during lead battery manufacture and during the production of studio glass. In these instances exposure is expected to be negligible. It is possible that it may also be produced at wastes sites where water or acid reacts with slag, dross or waste. However, such occurrences are expected to be uncommon.
10. During the handling of the gas for distribution, the three companies involved indicate that the dilution of the substance will be within a dedicated gas mixing area with workers wearing self-contained breathing apparatus and be limited to approximately once per year for between 30-40 minutes. Up to 3 workers per company would be potentially exposed in GB. Furthermore, the actual transfer and mixing process will occur in an enclosed gas cabinet with exhaust ventilation. Any leakage will be detected by gas detectors and will result in automatic valve shutdown, an audible alarm ringing so that the area would be evacuated. Workers would only re-enter wearing self contained breathing apparatus (SCBA). As the workers are wearing SCBA, exposure will be negligible.
11 As the use of hydrogen selenide as a dopant gas is within an enclosed system, industry believe that exposure could only occur during the changing of gas cylinders or in the event of a leakage within the system. Before the fitting and removal of cylinders, the system is purged and workers would wear SCBA. In the event of a leakage into the glove box from the reactor chamber where hydrogen selenide is deposited onto a wafer together with arsine and phosphine in the system, there would be automatic valve shutdown of all gas supply and an audible alarm would ring indicating a release of gas. Appropriately trained personnel with SCBA would then respond to the incident mitigating any gas release.
12. Overall, because the semiconductor industry and the gas distribution companies use state of the art technology to control any release of gas it is expected that potential exposure will be negligible. All companies contacted indicate that if a MEL is implemented no additional expense would be required to control to the proposed levels.
13. The OES for hydrogen selenide i.e 0.05 ppm 8-hour TWA has been withdrawn and in the meantime until a MEL is assigned to the gas, ACTS considers that the general requirement on employers under the Control of Substances Hazardous to Health (COSHH) Regulations 1999 will ensure adequate control of exposure to this substance.
14. It is proposed that the MELs will be based on the EU IOELVs of 0.02 ppm (8-hour TWA) and 0.05ppm (15-minute reference period) to control exposure because few new data on hydrogen selenide have emerged since the SCOEL review to warrant a higher or lower domestic limit.
15. Since potential exposure to hydrogen selenide is already strictly controlled by industry, it is unlikely that there will be any further reduction in exposure to hydrogen selenide. Health benefits due to implementing the MELs are, therefore, likely to be negligible.
16. In GB, hydrogen selenide is used by six companies as a dopant gas in the preparation of semiconductor materials for use in lasers, solar cells and other optoequipment. It is usually handled at a concentration of 1%. The gas is also handled by three gas distribution firms who supply it as a minor component in a mixture of carrier gas, usually hydrogen at a 1% to 5% concentration depending on customer specifications. Up to 30 employees may be potentially exposed to hydrogen selenide in these industries. No occupational exposure data was available from industry.
17. Hydrogen selenide may also be produced as a by-product where selenium compounds are present such as during alloy smelting but it is expected to be negligible because of the relatively trace amounts of selenium likely to be present in the parent selenium.
18. The costs of implementing a long term and short term MEL based on the proposed IOELVs or any other likely proposal will be negligible for the reasons given above namely it is only handled by nine companies in GB using state of the art control technology. In industries where it may be produced adventitiously exposure is likely to be negligible. Any high concentrations of the gas likely to be produced in the uncommon reaction of water or acid with waste material are expected to be rapidly dissipated by natural ventilation and the fact that hydrogen selenide is a strong reductant and decomposed quickly in moist air.
19. In those gas distribution companies which use alarm systems set at half the withdrawn OES (0.025 ppm 8-hour TWA) to detect leakage in their enclosed gas cabinets, there should be no additional cost to re-set the alarms at lower concentrations because this will be carried out by using their own staff. The semiconductor companies do not expect any additional monitoring for the gas if MELs are assigned as they believe the gas is tightly controlled.
22. Because of the small number of workers, the use of state of the art control methods by industry and the likely negligible exposure to the gas, it is not expected that the workload of HSE inspectors will increase because of the implementation of MELs for hydrogen selenide.
23. There should be no environmental impacts other than the potential health effects discussed above.
24. The information above was based on consultation with the three suppliers and the six present users. The following industrial associations were also consulted i.e. The British Glass Manufacturers Confederation, the Environment Service Association, the British Compressed Gases Association and Electronics Components Industry Federation and the Microelectronic Semiconductor Manufacturing Joint Working Group.
25. There is no reliable animal or human data available to establish a NOAEL for eye and respiratory irritation and so an OES is not appropriate. MELs are therefore, proposed based on the IOELVs of 0.02 ppm 8-hour TWA and 0.05 ppm 15-minute reference period.
26. Industry are confident that the implementation of MELs for hydrogen selenide will not impose an additional cost burden for the reasons given above; there should be negligible costs.
27. The MELs will be enforced at workplaces by HSE inspectors. The assignment of MELs will not result in an increase in workload for inspectors. The MELs will be revised if significant new information becomes available that places doubt on the current assessment of the health risks.