Pure as the Driven Snow

In my mid-twenties I worked as a production supervisor in a bakery near Sheffield. The production line I was responsible for produced tea cakes, white baps and hot cross buns for Marks and Spencer (other retailers of perfectly fine ‘morning goods’ are available). We made a lot of hot cross buns. And by a lot I mean we produced all of the hot cross buns for every M&S store in the UK.

The bakery products were all made using white flour. I like flour because I like the things that you can make with it. I like bread, I like cakes and I really like biscuits (except custard creams but that is another story from a different factory). Before I worked there I had only encountered flour in a domestic kitchen setting. What I had not appreciated was that flour dust is hazardous and that there are both safety and health risks associated with its use. These risks were explained to me, in no uncertain terms, as a part of my company induction. The most significant health risk is respiratory sensitisation (occupational asthma). The safety risk comes in the form of dust explosions – it will blow you away.

Wheat flour is made up of finely divided organic particles in a range of sizes. These particles can easily become airborne during handling. The larger particles will settle out of suspension under the influence of gravity. But the smaller particles will stay airborne for very long periods of time. When you throw flour dust into the air the larger particles are visible to the eye. The smaller particles will be invisible to the human eye under normal lighting conditions. It is only when the lighting conditions are arranged just so that they become visible.

Inhalation of flour dust causes irritation to the nose and upper respiratory tract. This irritation will be experienced by everyone to some degree. Repeated exposures can cause some people to develop respiratory sensitisation to the flour dust (a very strange reaction). Their respiratory system recognises the flour dust (more specifically certain organic molecules in the dust) as an allergen and an allergic response is provoked. This response involves constriction of the respiratory airways by muscle contraction and inflammation of the lining of the airways, excess mucus production, coughing and wheezing. In other words an asthma attack.

Once a person has occupational asthma then they are likely to remain sensitised to flour dust for the rest of their life. Therefore this is not a sensitisation that you want to develop and it may well spell the end of your job if it is not possible to avoid exposure to flour dust.

The other significant risk associated with flour dust is that of explosions. Create a dense flour dust cloud in air and ignite that cloud and the small particles will burn. A flame front will move through the cloud, igniting and combusting each particle in turn. This will happen quickly and is called a deflagration. If you want to see what an unconfined dust cloud deflagration looks like then take a look at this video news-report from the US.

Put the dust cloud in a container and the combustion process and heat released will cause a rapid rise in pressure inside the container that may cause it to fail. Boom! The container peels apart like a banana, a pressure blast wave is released into the local area, shrapnel flies everywhere and a burning fire ball of combusting dust, gas and soot escapes. And if that’s not exciting enough for you then the blast wave released by the primary explosion shocks any dust that might be resting on surfaces or inside other containers up into the air, where the dust/ air mixture can now be ignited by the advancing flame front leading to further deflagrations or secondary explosions. Boom ! Boom ! Boom!

Though dust explosions are not frequent events the wood dust explosion at Brosley Mill in 2015 is a reminder that the storage and handling of combustible dusts creates an ever-present risk.

For a dust explosion as described above to occur five factors need to be aligned: a combustible dust; an ignition source; oxygen; dispersions and containment. This is often referred to as the dust explosion pentagon (not pentagram – that’s an entirely different thing associated with the dark arts).

This is essentially the fire triangle with two added factors; dispersion, which refers to the mixing of the dust with air; and containment, which refer to the fact that the dust/ air mixture is inside some form of vessel or ductwork such as a silo, bin, hopper or duct.

For an explosion to occur the dust concentration would need to be at least 75 g.m3 (grams per cubic metre). This is a very dense flour dust cloud – it would be difficult to see your hand at the end of your arm at these dust concentrations. This is not likely to occur outside of a container under normal circumstances. But it is entirely likely and expected inside the storage silo when it is being filled, or inside a dust collector during normal operations. So the main risk of dust explosions arises inside dust handling equipment during normal operation.

The flour dust concentrations at which explosion risk exists are factors above those at which health risk exists. The Workplace Exposure Limit (WEL) for flour dust as given by EH40 is 10 mg.m-3 (milligrams per cubic metre).

The relevant legislation here is the Control of Substances Hazardous to Health Regulations (COSHH) for the health risk and the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) for the explosion risk.

There are, of course a range of preventive and mitigating measures available to combat both the health risk and the explosion risk. I won’t cover these here other than to say that I did get to see first-hand the explosion relief panel at the top of the 90 foot (27 metre) flour silo at the bakery one spring day. One of the hygiene staff asked me if I fancied climbing the fixed vertical access ladder up to the top of the silo just for a quick look around. I did a lot of rock climbing at the time so jumped at the chance. We went up the ladder and into the aluminium dome (a little hideaway) right at the top of the silo. As we entered the small space inside the other guy stopped me from stepping forward. In front of me was a large hatched off area. In the middle of the floor – which in effect was the top of the silo – was a large plastic explosion vent. Step on that and you could fall through and down into the body of the silo and onto (or into) the flour dust. This explosion vent was designed so that if a primary explosion did occur in the silo the build-up of pressure would cause the vent to fail before any other part of the container. A jet of burning dust and hot gas would then be exhausted safely up out of the silo, with no secondary explosions.

I look back on this little silo climbing escapade with great fondness. But also with a little alarm. I was a new starter at the factory. My guide never asked or told anyone what we were doing and we free climbed up the access ladder. This was before the Work at Height Regulations and Confined Spaces Regulations. But even so I think we were probably pushing the boundaries and should not have been up there. Still it was a good view; we were pretty high up there and I didn’t ever want to come down.

Dr Jim Phelpstead BSc, PhD, CMIOSH

RRC Consultant Tutor