Oily Rags, Solvent Vapour and a Locked Room Until Monday
Two students are finishing six turned bowls on a Thursday evening. The room has general ventilation but no extraction at the bench. They sand on a bobbin sander whose extraction bag has a tear, and brush dust off the bowls with a hand brush. They then wipe on boiled linseed oil and, after 20 minutes, brush on a solvent-borne sanding sealer from a 5 L can. A 2 kW fan heater is running because the room is cold. Used rags are thrown into an open plastic bin in the corner. One student has asthma; the other has eczema on her hands. A tin of solvent-based wood preservative has been left open on the bench. The extinguisher by the door is a CO2 type. The session ends at 21:00 and the room is locked until Monday morning.
1. Rank the hazards in this room and justify the order. Then give controls using the hierarchy of control, and say what you would fix before the next session.
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Ranking by likelihood × severity.
Solvent vapour plus an ignition source in a closed room. The sealer and the open preservative are releasing flammable vapour while a fan heater with a hot element is running a couple of metres away. Solvent vapours are heavier than air, so they pool at floor level, spread, and can reach the heater. In a room with only general ventilation this is a flash-fire risk, and it is the hazard most likely to injure someone tonight.
Oily rags left in an open plastic bin, with the room locked for three days. This is a fire that starts when nobody is there, which makes it the most severe outcome even though ignition takes hours.
Airborne dust from a torn extraction bag plus dry brushing. Fine dust from sanding is a respiratory sensitiser and a known asthma trigger; it also settles everywhere and is re-lifted. The fire and explosion risk from dust is low at this scale, but the health risk to an asthmatic student in the room is immediate.
Skin contact. Solvents and oils on already-compromised skin cause dermatitis; the eczema makes absorption worse.
The open preservative tin. Vapour, spill and contamination, and it is unnecessary to leave it open.
Controls, in hierarchy order. Eliminate: do not run solvent finishing in a cold room in the evening - schedule it for a ventilated daytime session, or drop the solvent products from the scheme. Substitute: water-based sanding sealer and a low-solvent hard wax oil in place of the solvent sealer and boiled linseed oil. Engineering: repair or replace the extraction bag, fit extraction at the sanding point and at the lathe, keep the room warm enough that a door or window can stay open without the heater. Administrative: no ignition sources during finishing, decant only what is needed into a small lidded container, a written safe system of work, supervision, a closing-down checklist, a fire blanket and a suitable extinguisher for flammable liquids - CO2 will knock down a small solvent fire but has little cooling, so add foam or dry powder and know which to use. PPE last: nitrile gloves, and respiratory protection only as a backstop.
2. Explain the mechanism behind the rag hazard and state exactly what must happen to those rags before the room is locked.
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The mechanism. Linseed oil is a drying oil: it does not evaporate, it cures by reacting with oxygen in the air, and that reaction gives off heat. On a flat surface the heat escapes as fast as it is made. Crumple the rag, though, and you have created a large oiled surface area inside a small, insulated mass of cloth, so the heat cannot escape. The rag self-heats, the reaction speeds up as it gets warmer, the cloth chars and eventually breaks into flame - often hours later, after everyone has gone home. It is a genuinely classic workshop fire and it happens in bins, in bags and in vehicles.
What to do before locking up. Never leave oiled rags in a plastic bin. Either lay each rag out flat, singly, on a non-combustible surface outdoors so the heat can escape; or immerse them in water in a lidded metal container; or place them in a self-closing metal safety can. Then empty the bin, check the bench and the floor for stray rags and for drips of oil on shavings or sawdust, and have the last person out confirm it. Rags must not be taken home in a bag or left in a car boot. Add a line to the closing-down checklist so it is not a question of memory, and use the same discipline with any oil-based finish, including tung oil and alkyd varnish.
3. The asthmatic student asks whether a dust mask will let her carry on, and the student with eczema asks about gloves. Answer both, with reasoning rather than reassurance.
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The dust mask question. Respiratory protection is the last line of defence, not a licence to enter a hazardous atmosphere. It only works if it is the right type, fit-tested to that person's face, and worn properly for the whole exposure - and even then it protects only the wearer, while everyone else in the room still breathes the dust. More importantly, tonight's exposure is not only dust: it is solvent vapour, and a nuisance dust mask does nothing against vapour. A half mask with the correct combination cartridge would be needed for vapour, which tells you the control is in the wrong place. The right answer for an asthmatic student is engineering and substitution: fix the extraction, stop dry brushing in favour of vacuuming with a filtered machine or damp wiping, and move to a water-based finish so there is no solvent vapour to protect against. If solvent work still has to happen, she should not be the one doing it, and the school should review the risk assessment with her and her parents. If she is ever asked to wear RPE, it must be face-fit tested.
The gloves question. Dilute solvents and oils will defat and irritate skin, and eczema already means a broken barrier, so gloves are sensible - but only the right ones. Nitrile gloves are the usual choice; latex is best avoided because of sensitisation. Thin disposable gloves are attacked by solvents, so they need to be changed as soon as they are contaminated, not worn all evening, and hands should be washed and dried afterwards. Barrier cream helps for incidental contact but is not a substitute for gloves. The product safety data sheet will list the recommended glove type and the exposure controls, so read section 8 rather than guessing from the label, and check the hazard pictograms before the tin is opened.