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MMD > Archives > August 1996 > 1996.08.14 > 06Prev  Next

Environmental Safety
By Jim Heyworth

> Robbie Rhodes write:
> Jim Heyworth: could you please write about the philosophy and practice of
> static electricity control in the hospital. Also, what "hospital grade"
> tubing might be suitable in piano repair, and what materials would be
> unsuitable. Also, what are the potential medical hazards of the Open
> Mercury Switch?

Since I've been retired for a few years I may not be totally up to date, but here goes.

The main reason for control of static electricity in hospital operating rooms has now all but vanished along with the use of flammable anaesthetics. The worst offender was cyclopropane.

/ \
H2C ----- CH2

This, supplied in a compressed-gas cylinder, used to be an almost universally-used anaesthetic. It was popular due to its very good analgesic properties which were particularly useful in obstetrics where it was possible to obtain excellent analgesia without unconsciousness. The fact that it was a gas eliminated the need for a vapourizer and messy liquids.

Unfortunately it is heavier than air, and as a result it was quite possible to inadvertently have explosive concentrations present in the operating room particularly at floor level. That is why the OR floors were made conductive (usually a special form of terrazzo) and the boots worn by the OR staff had conductive soles connected by conductive straps to their ankles. There was even a device that looked like a medical office scale onto which the wearer of this attire would step and a meter would inform him/her if he/she were sufficiently conductive. The mattress pads, anaesthetic tubes, face masks, etc. etc. were all made of conductive material so that static sparks would not be generated and accidentally cause an explosion.

Most often this conductive material was either black rubber (containing carbon black) or black rubberized cotton. I am not aware of any clear plastic materials that were deliberately made conductive, although I am not an expert in this area.

Nowadays most hospitals have banned all flammable anaesthetics from their operating rooms and so static control is not anything like the concern it used to be. The conductive boots have gone, although I seem to recall that the mattresses are still black and rubberized. They are probably conductive, but not necessarily required to be so. The hoses to the anaesthetic machines are now transparent plastic, instead of black rubber, and non-conductive. Electrosurgical units, which create showers of sparks, are used during practically all procedures, the main concerns with them being proper grounding of the patient to the high-voltage RF generator to avoid causing burns to him/her at sites remote from the surgical area of interest, and interference with monitoring equipment.

Just for fun I got out the old ohmeter and tried out a few samples of player piano tubing. Many were non-conductive even to 1000 megohms, but a few had as little as 1000 ohms across the wall. Two thicknesses of this (2000 ohms) would consume about 12 watts at 115 Vac and so would become very warm indeed. Once they started to carbonize they would become much more conductive and could very easily start a fire.

After all this screed, probably the important point to take home is that the only things that should be used as electrical insulators are materials designed for the purpose. Anything else, particularly at line voltage, is Russian roulette.

With regard to other properties making hospital grade materials suitable/unsuitable for pneumatic work, that is left as an execise for the student. I have a small piece of plastic non-conductive anaesthetic hose in my Duo-Art where it goes around a corner where nothing else would fit and has performed well for 20 some-odd years. On the other hand I have seen surgical rubber and some plastic tubing deteriorate rapidly to a sticky goo.

Now, on to the mercury question.

The main concern here is contact with mercury vapour. When metallic mercury is lying quietly cool in a dish in a ventilated area it is not particularly hazardous. The problems that we hear about regarding the ingestion of mercury being poisonous really refer to _compounds_ of mercury which can be very hazardous. The ones we seem to hear about most recently apply to children exposed to low concentrations over long periods.

There have been numerous accidents where people have ingested fairly large quantities of metallic mercury and suffered few, if any, ill effects.

The story is quite different if metallic mercury is spilled and allowed to spread out, particularly if it becomes heated to the point of evaporation. (Easy to do if it ends up on a steam pipe or other hot object like a light bulb.) Air saturated with mercury at 20 deg. C. contains about 15 mg/cubic metre and at 40 degrees, 68 mg/cubic metre. I don't know what the current figures are (they tend to change as measurement methods and experience improve) but the reference I have at hand indicates that a level of 0.1 mg/cubic metre is considered the point at which the vapour becomes hazardous.

So, if a mercury switch arcs there could be a puff of vapour created which, since the concentration in air is well below saturation, will hang around. A number of switch closures could cause hazardous levels to accumulate if ventilation is poor. If forced-air ventilation is used the hazard is likely to be considerably reduced, although ventilation of the room itself must be taken into consideration. Since the switch is likely to be in an enclosed, probably warm, poorly-ventilated space, I would be inclined not to use such a switch. If I absolutely had to, I would pay very careful attention to ventilation.

If a room-temperature spill occurs there are chemicals available to neutralize it, although if it is small and one is very careful to maintain adequate ventilation, sweep up _all_ of the metal and put it in a sealed jar the hazard of doing so is not great. I'm sure quite a number of us (including me) have at one time or another changed pennies to dimes with a little mercury and suffered no effects as far as we could tell.

The answer to the anticipated argument that these switches were used for years with nobody suffering ill effects is, we really don't know that they didn't. At the time these devices were prevalent the hazard was barely recognized and therefore not sought out. Test methods were slow and tricky to use. So not sought out, not found and therefore not reported. Not reported does not mean nothing happened. As a matter of fact, how many people using devices with open mercury switches would even think of them as being a cause for some peculiar complaint ? How many doctors today without an obvious reason to do so would ask a patient if he/she had been exposed to mercury vapours, or even consider testing for the metal ? Certainly not likely on the first visit!

Jim in Sechelt

[ (Postscript from Robbie: The expression "Mad as a Hatter," and the
[ Mad Hatter character in "Alice in Wonderland", are the result of the
[ method once employed to cure animal pelts for hats: mercury vapors.)

PPS. It is my understanding that the Mad Hatter's problem was caused by soluble mercury _compounds_ used in the manufacture of felt and not mercury vapour. These compounds were transferred to his fingers when he handled the material and eventually to his mouth when he licked his fingers as one might expect him to do while sewing.

(Message sent Wed 14 Aug 1996, 17:50:38 GMT, from time zone GMT-0700.)

Key Words in Subject:  Environmental, Safety

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