Monitoring for toxic gases

UK Health and Safety Executive (HSE) and COSHH Regulations

Chemicals, fumes, dusts and fibres can under many circumstances have harmful effects to workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Persons exposed to harmful substances may develop illnesses (for example, cancer) many years after the first exposure. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

It’s worth noting that most flammable gas hazards can potentially occur when the concentration of gases exceed 10,000ppm (1%) volume in air or higher. Toxic gases typically need detecting in sub-100ppm (0.01%) volume levels to protect personnel and often at concentrations sub 5ppm.

In the UK, under the Control of substances hazardous to health regulations 1999 (COSHH regulations) the Health and Safety Executive (HSE) sets occupational exposure limits (OELs) and publishes these in a document entitled EH40. These lists have legal status and similar legislation exists elsewhere; COSHH takes into account the European Commission Directive 80/ 1107/EEC. COSHH covers all toxic substances except those which have their own legislation (asbestos, lead, radioactive materials and materials present in mines).

The regulations stipulate requirements for employers, and in a few cases employees (failure to comply is subject to the penalties of the Health and Safety at work act 1974). The requirements are:

• Design and operate processes and activities to minimise emission, release and spread of substances hazardous to health.
• Design and operate processes to minimise human interaction within potentially dangerous environments.
• Take into account all relevant routes of exposure, inhalation, skin absorption and ingestion, when developing control measures.
• Control exposure by measures that are proportionate to the health risk.
• Choose the most effective and reliable control options which minimise the escape and spread of substances hazardous to health.
• Where adequate control of exposure cannot be achieved by other means, provide, in combination with other control measures, suitable personal protective equipment.
• Check and review regularly all elements of control measures for their continuing effectiveness.
• Inform and train all employees on the hazards and risks from the substances with which they work, and the use of control measures developed to minimise the risks.
• Ensure that the introduction of control measures does not increase the overall risk to health and safety.

The assessment is performed by the employer with help from the HSE if needed. The best way of controlling a risk is to prevent exposure but if this is not possible, a process may have to be enclosed or ventilation and extraction equipment used, or special handling procedures employed. It should be possible for all people to work in a safe environment day after day and HSE publishes Guidance Note EH40 to help employers to control their processes adequately so that workers are not exposed to levels of toxic materials above the recognised safe levels.

The monitoring aspect of COSHH is particularly relevant to Crowcon’s products where monitoring is required:

• If the failure of control measures would lead to serious health risks
• If it is not certain that exposure limits are not being exceeded
• If it is not clear that control measures are working properly

When monitoring of toxic gas exposure is required employees must be told about potential risks and precautions to be followed. The results of any monitoring and health surveillance should be recorded.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless and are harder to physically avoid than liquids or solids. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Crowcon’s toxic gas detectors and their accessories have been designed with this in mind, and the need for continuous monitoring and recording has led to the development of data logging facilities.

There is increasing emphasis on Environmental Monitoring in the workplace. It is recognised that employee’s health and well‑being may be affected by pollution from industrial processes, traffic fumes and the decay of waste. Levels of NOx (oxides of nitrogen), SOx (oxides of sulphur) and increasingly; CO₂ are being monitored to quantify exposure.

The 2005 issue of EH40 introduced new terminology for defining occupational exposure limits (OELs). The previous system defined OELs as maximum exposure limits (MELs) and occupational exposure standards (OESs). MELs and OESs have been discontinued and replaced by a single type of OEL known as the workplace exposure limit (WEL). The numerical values initially remained the same, but some have since been lowered as new information has become available. The OESs for around 100 substances have been deleted as the substances are now banned, scarcely used or there is evidence to suggest adverse health effects close to the old limit value.

From 1989 to April 2005, Occupational exposure standards were in two categories.

Maximum exposure levels (MELs) were for the more dangerous substances which may cause the most serious health effects (such as cancer or asthma) and exposure to materials with MELs were kept as low as possible and certainly not above their MEL.

Occupational exposure standards were set at a level at which there was no indication of risk to the health of workers and employees exposed by inhalation day after day.

As mentioned above the new workplace exposure limit (WEL) list will combine OELs and MELs using the same numerical values. the list gives long term (8 hour) exposure limits (LTELs) applicable to exposure during a normal working day and short term (15 minute) exposure limits (STELs) applicable to occasional exposure to higher levels. Therefore, WELS are concentrations of toxic substances in the air, averaged over a specified period of time and referred to as the time weighted average (TWA).

WELs can be expressed as parts per million (ppm) and milligrams per cubic metre (mg/m3) if the substance exists as a gas or vapour at normal room temperature and pressure. Compounds that do not form vapours at room temperature and pressure are expressed in mg/m3 only. Refer to the Detector Calibration section of this document for information on converting WELs expressed in PPM to mg/m3.

When mixtures of toxic gases are encountered the effects on health are often additive and this needs to be taken into account (exposure to two gases with similar effects, each at 50% of their WELs may be equivalent to working at a WEL or the two gases together may have an enhanced effect). There is a detailed explanation of Mixed Exposures in EH40/2005.