Dust cloud

Contaminant clouds – more than meets the eye

Contaminant clouds – more than meets the eye

Many workplace activities can produce clouds of airborne particulates like dust, fume and smoke. When you think of a dusty workplace you may picture stone being polished, bricks being cut, or sweeping up a factory floor – a cloud of fume perhaps coming from engine exhaust or smoke from a thermal degradation process.

But your eyes only give you part of the story. Particles in a contaminant cloud often do not travel alone. And it may not necessarily be only the particle itself which causes the potential for ill-health effects. Other substances may be present in this ‘dust’ cloud. For example, by-product gases like carbon monoxide and ozone can be generated alongside the volatilised metal fume when welding. Colophony produced from rosin-cored flux fume is the main exposure concern when soldering, with the metals being less likely to volatilise into fume at the relatively low temperatures of soldering. Diesel engine exhaust emissions, asphalt fume, plastic fume and many other combustion or thermal degradation type processes produce a contaminant cloud of particulates and a slurry of other airborne hazards.

Judging a contaminant cloud based on its visible ‘dustiness’ can therefore be misleading. What appears to be a cloud of particulates may be laden with hazardous gases and vapours. And what’s more, the dust itself may not be fully visible to the naked eye. A particulate cloud created when, for example, cutting stone can appear dusty. But silica dust particles can be so small that they are not visible under normal lighting and the worker may unknowingly be exposed to even higher airborne concentrations than at first perceived.

Dust exposure can lead to a range of occupational diseases, depending on the size and nature of the particulate and the extent of exposure. Additionally, the other hazardous substances potentially within this contaminant cloud may also contribute to its hazardous nature. Which can lead to respiratory diseases if adequate exposure control measures are not implemented. It is the employer’s duty to understand fully what hazards the employee is facing and how to adequately control this exposure. In this article we will be talking about contaminant clouds- when is there more than meets the eye?


Part 1 – Diesel Engine Exhaust Emissions

Many industries use diesel-powered vehicles and equipment. When we picture engine exhaust we think of a cloud of smoke coming out of the tailpipe. But there is more to this than the visible particles. Diesel engine exhaust emissions (DEEE) contain a complex mixture of gases, vapours, liquid aerosols and particulates which are the products of combustion. The major constituents are nitrogen, carbon dioxide, steam, carbon monoxide, oxides of nitrogen, and soot (carbon particulates) which makes up a significant portion of the emissions.

The soot of DEEE tend to form chain or cluster aggregates onto which many substances may adsorb. Trace quantities of unburned or only partially oxidised diesel fuel can results in the generation of aldehydes, polycyclic aromatic hydrocarbons and other various hydrocarbons. The composition and even the colour (white, black, blue) of this exhaust cloud can vary depending on the engine’s state (mechanical faults causing the release of oil or unburnt fuel), on the age and type of engine, fuel type and exhaust filter, and on the workload demand on the engine. Research shows sufficient evidence that prolonged heavy exposure to DEEE is associated with an increased risk of lung cancer which is likely due to fine exhaust particles and the hazardous substances absorbed onto them.

DEEE exposure can also cause irritation, fluid build up in the lungs, and asphyxiation in extreme circumstances. And so judging the health risk of a contaminant cloud by its visible ‘dustiness’ does not necessarily correlate to its true risk as unseen hazardous substances can be present and at potentially elevated levels.

Diesel particulates are extremely fine with very little mass. Once airborne it behaves very much like a gas and stays airborne for a long time. When measuring (if necessary) for DEEE exposure, occupational hygienists would at first concentrate on the diesel particulate (active sampling with elemental carbon analysis) as this is so fine that it is fairly representative of the other gases and vapours associated with DEEE.

If the elemental carbon concentration is high then it is likely the concentration of other exhaust gases and vapours will also be high. With the exception of carbon monoxide (CO) and nitrogen oxides (NOX) as there is no direct correlation between these gases and the elemental carbon concentration. Therefore the concentration of these gases (CO and NOx) must be determined independently using another method such as a direct-read data-logging device (HSE, RR252, 2004). The DEEE cloud can be sampled for the other suspected contaminants such as aldehydes, polycyclic aromatic hydrocarbons and volatile organic compounds. But this is normally carried out following an initial sampling exercise with the elemental carbon used as a surrogate (alongside the CO and NOx monitoring). Sampling aside, the focus though should of course be on preventing exposure to DEEE or controlling exposure if unable to prevent. Control measure selection must be made following the hierarchy of control and can include ventilation, personnel distancing, staff rotation, improved work practices and engine maintenance.

If we do sample for DEEE exposure, what will the results be able to tell us? What do we compare against? The topic of DEEE exposure limits is debated. Some governing bodies recommend that workers should not be exposed to DEEE levels greater than those experienced by the population in urban areas.

Also, limits may vary based on the specifics of those research parameters. This shows one of the problems with assessing DEEE as individual compounds, as it is the mixture of the hazardous constituents which may cause ill-health. That being said, workplace exposure limits (WELs) are available for CO and NOx under the HSE’s EH40/2005 ‘Workplace Exposure Limits’ document. A WEL is not set for the particulate component of DEEE however (i.e. the soot represented as elemental carbon). Although a limit of 0.05 mg/m3 (8-hr TWA) may be implemented in the next couple of years. This would be set for elemental carbon from all diesel engines, without distinguishing between sources of diesel emissions.

Some sectors may find this limit difficult to achieve. And so it is likely that a transitional period would be introduced before the limit is applied. WELs are available for several individual compounds as well which may be present in the DEEE cloud e.g. naphthalene, acrolein, formaldehyde. Although these WELs and guidance limits give us a figure to compare against, we must keep in mind that there are no safe levels of exposure to a carcinogen and the law requires that exposure must be kept to as low a level as is reasonably practicable. This would be achieved through implementation of adequate controls and maintaining the performance of those controls.