Two decorators, one died and one seriously burnt in a blast caused by the ignition of varnish fumes inside a domestic flat at Ap Lei Chau when they returned to the flat to monitor the progress of the decoration work. At the time of the accident, the floor in the flat had just been vanished. All windows and doors in the flat were closed and sealed to keep the vanished floor free of blemishes – a common practice in the decorating industry to prevent circulating air from making surfaces of varnished floors uneven. A short while after the two decorators had entered the flat, an explosion occurred. The two men suffered serious burns and one died from complications subsequently at Queen Mary Hospital.

Analysis

The vanish paints used in floor polishing contain volatile solvents that are highly flammable and toxic. The flammable vapor from the solvent evaporates readily and accumulate in the surrounding atmosphere. When all windows and doors are closed, the vapor can quickly accumulate to a concentration (the lower explosive limit, LEL) that can be ignited by a single spark. Subsequent investigation by the authorities revealed that the ignition may have been caused by the smoking of cigarette by the workers. In fact, the explosion could also be caused by sparks generated by electrical equipment and switches.

Lesson Learned

Fire and Explosion Hazards

Extreme care must be taken when handling flammable liquids, especially volatile ones which give off vapor readily under room temperature. The volume of space for evaporation and the degree of ventilation are very critical in determining the risk of explosion. The air volume inside an air tight flat can easily be filled up by the flammable vapor given off from the floor vanish spreading over a large surface area. When the concentration of the flammable vapor reaches its LEL (lower explosive limit), it can be readily ignited by any ignition sources (can simply be sparks from electrical equipment, door bell, switches etc.), and it can burn violently with an explosion.

The practice of sealing all windows and doors after floor vanishing processes should be reviewed by the industry. This is an extremely hazardous practice which also poses a high risk to the neighboring flats.

Although the domestic flat is a habitable place, with the hazardous substances and given the conditions described above, it should be treated as a "confined space" with all the necessary precautionary measures, such as monitoring the atmosphere conditions with scientific instruments. Currently, entering and working in confined spaces are controlled by the Factories & Industrial Undertakings (Confined Spaces) Regulation. A "permit to work" procedure is also established in the HKUST for working in confined spaces.

Health Hazards

The solvents can also cause serious health hazards when inhaled and contact with skin. Workers doing the floor polishing work should wear appropriate respirators and protective gloves. Good ventilation should be maintained during the operation to reduce the concentration of the toxic vapor.