Monitoring for welding fume

Fusion welding processes can be grouped according to the source of the heat, for example, electric arc, gas, electrical resistance and high energy. All these processes generate fumes to a greater or lesser extent and according to EH40 ⁽¹⁾, fume is a word that is often used to include both gases and vapours. This is not the case for occupational exposure limits where ‘fume’ should normally be applied to solid particles (dust) generated by chemical reactions or condensed from the gaseous state, usually after volatilisation from melted substances, as is the case with welding.

The breathing of welding fumes has long been associated with chronic respiratory problems, including bronchitis, asthma, pneumonia, emphysema and pneumoconiosis. It has been common practice, therefore, to carry out a risk assessment by undertaking workplace air monitoring in the welder’s breathing zone (a 30cm radius centred on the nose & mouth) for some of the common metal oxide compounds and gases, namely:

- Iron oxide

- Manganese

- Chromium VI & III

- Nickel

- Ozone

- Nitrogen Monoxide and Dioxide.

The accepted method for sample collection, employed for the last six decades, is to draw a known volume of air through a suitable medium housed in a sampling head using a sampling pump followed by appropriate laboratory analysis. For dusts and fumes this would be a filter paper or solid media and for gases and vapours, through a sorbent tube. 

In the case of welding fumes, the dust is collected on an MCE filter which can be analysed directly without further sample preparation by X-ray fluorescence spectrometry. The type of sampling head depends on the size fraction of interest i.e. inhalable or respirable. Advice on the method for sampling and analysis is provided by the HSE ⁽²⁾ in the UK and OSHA ⁽³⁾ and NIOSH in the USA.

Interestingly, the basics of sampling with a pump haven’t fundamentally changed much in 60 years. Lithium-Ion batteries and Bluetooth connectivity are recent technology developments along with improvements in digital flow calibrators that all improve the equipment user’s productivity and increase the successful outcome of a sampling campaign. 

It is very important to check that the sampling pump meets the latest standard i.e. ISO 13137 ⁽⁴⁾. In particular, if using a cyclone head for gravimetric sampling, don’t assume that the pump has sufficiently low pulsation, which the ISO standard states shall not exceed 10% of the flow rate. A large pulsation value means that the size cut performance of cyclones can be affected because their performance is flow rate dependent. In addition, less sample is collected using pumps that generate significant pulsation ⁽⁵⁾. 

To ensure that the pump is performing to specification it is important to check that it has not exceeded its recommended run time between services, typically 2-3000 hours. In the field, it is good practice to check the pump’s flow rate calibration in a clean environment both before and after use each day using representative sampling media which should be retained for control purposes.

Hexavalent chromium and nickel are already defined occupational carcinogens, however, in the UK, the Health and Safety Executive (HSE) released a safety alert in February 2019 (6) highlighting new scientific evidence from the International Agency for Research on Cancer suggesting that exposure to all welding fumes causes lung and kidney cancer.  Following the findings, the Workplace Health Expert Committee has endorsed the reclassification of mild steel welding fume as a human carcinogen.  Mild steel is the most commonly welded metal used in both manufacturing and construction.

Local exhaust ventilation

According to the HSE, at present, general ventilation does not achieve the necessary control. HSE is therefore set to strengthen its enforcement expectation on all welding fume, including mild steel welding. This means using effective engineering controls such as local exhaust ventilation (LEV) supplemented by suitable respiratory protection equipment (RPE) if there is any chance of residual fumes (which can be confirmed by personal monitoring) but RPE must be worn when welding outdoors. A typical LEV system comprises a hood, ducting, an air cleaner or arrestor, an air mover, and discharge.

With a mechanical shaker type LEV unit, unclean air is drawn through the unit by a fan and over filter bags which trap the contaminants. The filter bags are shaken by a motor and the particles fall off into the dust collection bin. The clean air is then drawn through the fan chamber and let to exhaust, either by ducting to the outside, or back into the workplace. If the unit exhausts back into the workplace, a secondary HEPA type filter is needed to ensure the quality of the returned air.

There are different filter bags available to fit into the LEV unit, dependent upon the particular contaminant. They can be of a different pore size (down to around 5µm) or treated for oil or heat resistance for instance. The filters wear over time, the seals on the bags may degrade, they get clogged and they become less efficient.  If there is significant dust present on the ‘clean side’, this is a clear visual indicator that the filters need changing. 

However for routine checks, you would not rely on just a visual inspection but ensure that the system is working efficiently by testing the ‘clean air’ leaving the outlet for levels of dust using a suitable measuring instrument.

Real-time dust monitors

Hand-held, data logging instruments for the real-time detection of dust, fumes and aerosols are available. As it is real-time, instant decisions can be made with regards to the effectiveness of controls and required intervention.  Real-time instruments wont tell you the composition of the dust, such as the ammount of metal in fumes, but can be highly effective at showing the effectiveness of control measures, and how these may change on a day to day basis.

Whilst the process of interest is ongoing, placing the probe close to the outlet and view in real time the data will give you a clear idea of the efficiency of the filtration compared to blank, ambient readings. To check for residual fumes use the, monitor the area around the worker. This will provide quantitative data to enable decisions to be made and assess the situation armed with more information. Is the positioning of the task and the worker to the LEV adequate for instance? 

The heightened concern over welding fumes and in particular of mild steel places a significant new challenge on many diverse sectors to ensure that the workforce is protected through effective control methods. It is vital that LEV is regularly maintained and workplace ambient levels are routinely monitored using quantitative methods such as real-time measurements and/or routine personal sampling to ensure a ‘belt and braces’ route to compliance.

References:

(1) EH40/2005 Workplace exposure limits, 4th Edition

(2) MDHS 91/2 Metals and metalloids in air by X-ray fluorescence spectrometry

(3) OSHA ID-125G Metal and Metalloid Particulates in Workplace Atmospheres (ICP Analysis)

(4) ISO 13137:2013 Workplace Atmospheres: Pumps for personal sampling of chemical and biological agents: Requirements and test methods.

(5) Anderson et al 1971, Lamonica and Treaftis, 1972, Caplan et al 1973, Blachman and Lippmann 1974, McCawley and Roder, 1975.

(6) http://www.hse.gov.uk/safetybulletins/mild-steel-welding-fume.htm#utm_source=hse.gov.uk&utm_medium=refferal&utm_campaign=welding-alert&utm_content=cross-site-banner