Tuesday, August 7, 2012

Mercury spill

I dropped a mercury thermometer a few hours ago. At first I felt glad as I thought it had survived, since I didn't hear glass breaking. But that was just an illusion: the glass surrounding the bulb is (no, was!) so thin that the breaking glass wasn't audible above the thump on the floor. It took me a few moments to realize that the bulb was no longer attached to the thermometer, and then it dawned on me that I would be spending the next hour or so cleaning up the many droplets that had dispersed over my floor.

Mercury droplets dispersed over poorly fitting tongue & groove wooden floor

After banning my dog to my bedroom, I set to work with a sheet of paper folded into a shovel (so the mercury wouldn't simply roll off the back end) and an artist's pain brush. It's surprisingly tricky to get small droplets of mercury to coalesce; I wonder if that is due to dirt getting picked up as they roll along. So it took me a while, maybe an hour or two. It really isn't comfortable kneeling on a hard floor for that long, crouched closely over the floor to see even the smaller droplets. (Small enough not to be visible in the photo above.)

After collecting the droplets, I swept the floor like I usually do (but far too rarely to be called a civilized person) when guests are coming to visit. Then a wiping with a damp cloth. I'm not sure the last two steps are very effective, but it didn't seem like an outright bad idea either. I forgot to apply the sand trick the poison info center lady told me a few years ago [1]. What would have been a bad idea is to have used the vacuum cleaner: that may have pulled the droplets from the floor, but subsequently they'd be atomized and/or heated by their proximity to the motor, thereby creating a fine mist of mercury droplets suspended in the air, and/or dangerous levels of mercury vapour.

Time for some numbers. Is the rest of the mercury, that part that I inevitably failed to remove, a realistic threat to my health? I think not. Here's why.

I'm pretty sure I got most of the mercury. Let's be conservative and estimate that 10% of the circa 1g spilled remains - 100mg. It's winter, so the indoors temperature is at most about 20°C. At that temperature, the vapour pressure of mercury is about 0.7Pa. If I make the house airtight, and wait long enough for the mercury vapour to saturate the air inside, I'll have at most 62mg/m3 in the air. That's clearly far too high to be safe. But I only have 100mg, and I have about 250m3 of air inside the house. A more appropriate number then is the concentration if all 100mg were to evaporate: 100mg/250m3 = 400µg/m3. That's still high if it were a chronic exposure, but my house isn't airtight. If it were, I'd suffocate.

I normally leave at least one door open the whole day. That way my dog can regularly report on what critters she successfully chased off our territory, and how far her tunnel to South America has progressed. So I'm probably getting somewhere around a complete air change every day, even on a windstill day, just from diffusion, convection, and eddies from my and my dog's movements. Mercury's evaporation rate at 20°C is 56µg/hr.cm2 (Another source has 7µg/hr.cm2. Let's use the higher rate.) To reach 10µg/m3 (above which the mercury vapour concentration is unsafe for occupational exposure [2]), 2500µg would have to evaporate every day. (And that would deplete the reservoir of mercury in 40 days, since I probably spilled no more than 100mg.) For that rate of evaporation to occur, there needs to be an exposed mercury surface area of 1.86cm2.

Now it gets difficult to estimate things. How finely is the remaining mercury dispersed? I probably cleared up everything with a diameter bigger than 0.5mm, smaller droplets rapidly becoming too small to see. Smaller is worse now, because a smaller droplet has a greater surface area proportional to its volume, than a larger one. But there's a limit to how finely the mercury could have dispersed, and I don't know what that limit is. Let's just guess, and assume that all the remaining mercury is in the form of droplets 0.1mm in diameter. Ouch, that leaves me with 4.6cm2. That's just enough to be worrying: it's about twice the "safe" exposed area of 1.86cm2, so after the daytime air change (during which I'll assume the equilibrium concentration is close to zero) when I close the doors for the night, the mercury vapour concentration will slowly ramp up to unsafe levels over the course of the night. Just as the concentration (at worst!) reaches unsafe levels, I'll be waking up anyway, only to open the door again, quickly clearing the house of any accumulated mercury vapour.

Overall, I conclude I'm not in any serious danger. I'm not the first to have broken a mercury thermometer, and I won't be the last. I've made what I think are conservative estimates, in one case even order-of-magnitude conservative. If that only just brings me to the threshold of danger [2], and then only for the short time that I'm pouring muesli into my breakfast bowl compared to a whole night of sub-unsafe levels, I'm not going to lose any sleep over this. I will make a few concessions to hydrargophobia: I'll leave a carpet over the spill site and likeliest area of lurking mercury, overnight (to slow the evaporation), and I'll be a bit more aggressive with air change rates.

[1] A few years ago when I was still living there, my mother dropped a mercury thermometer that had been in the fridge. Freaked out a little, I called the poison info center based at the Tygerberg hospital, and the kind lady reassured me that metallic mercury isn't all that bad, that it's only when in vapour form or as compounds with other substances that it gets horrifying. Her advice was to sweep it up like dust, adding some sand to make it easier to get the droplets of mercury onto the shovel.

[2] I'd have to have trouble holding down a job if 40 days is to be a typical duration. That said, I am not currently employed. Here, though, the population average matters, not my individual circumstances.

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