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.

Saturday, August 4, 2012

Owning a home is sometimes like owning a white elephant

If you're considering buying a house, don't let yourself get carried away by the romance of owning the roof above your head, of being regent in your own castle. Sure, that's all a nice position to be in, but it isn't free. You get to choose how to keep the forces of entropy at bay: pay someone to do it for you, or DIY. I choose DIY, as that's the only ideologically acceptable option for a stingy bastard like me.

This post may sound a bit whiny, and you may be tempted to roll your eyes and mutter, "First world problems!" This is true - I wouldn't seriously consider giving up living in a house even if the workload were doubled. In fact, I like the unkempt look and wouldn't be too troubled by conceding some battles to the forces of entropy. My reasons for doing yard work at all have more to do with enjoying the meandering workout that it gives me than with having a home magazine-worthy garden. Think of the whining here as hyperbole. I'm looking for things to complain about in this post.

A tangle of branches and stems cut from my Brazilian pepper tree. A season or two ago the Wisteria would have been the source of a tangled heap like this one.

I have at least three distinct compost heaps in my garden. This one is probably fastest, likely due to the fact that I feed it clippings from my shredder, and probably also due in part to my feeding it a dilute solution of urea at night. It's nice not to have to buy compost, but they do take up a fair amount of space.

I collect heaps of plant matter on my stoep until I have enough for a session with the shredder. The two far heaps will go on the compost heap pictured above, while the one next to my dog will require some alternative treatment. They're cuttings from my lavender bushes, and they smell quite strongly of camphor, which I suspects upsets the critters in the compost heap. (An earlier batch of lavender cuttings took forever to rot to an appreciable degree.)

The Brazilian pepper tree branches that are too thick to put through the shredder become firewood after a year or more. (It takes a while for the wood to become dry enough to burn without excessive smoke.) It's surprising how much firewood a few pruned branches can yield.

My driveway is covered with pebbles, but over the years (starting before I moved in, four years ago) the pebbles have become compacted into the soil below. It's likely that the principal cause of this is the lack of a plastic sheet below the pebbles. As a result, weeds (and other welcome, but unintended plants) grow in the middle where car tyres don't squish them.

The Brush Cherry on the left likes to scrape against my car, so I have to prune it back now and then.

The gate motor has been giving trouble since the start. Its favourite party trick is to open the gate, then to refuse to close it again for several minutes, and when it finally does, to give up halfway and claim that the battery is "faulty".

The wooden postbox swells in winter, so as to become difficult to open by pulling on the sliding bolt. The handle, of course, has broken off years ago when I had left the postbox door open to dry, and then absent-mindedly opened the gate, ramming the handle and bending the bolt holding it to the door. When I tried to bend the bolt straight, the handle broke right off.

My plum tree has suffered a sever split sometime in its lifetime. Judging by its stem it must be a rather old tree - certainly several decades old. One stem fell to the left in this picture, where it seems to have rooted and become essentially a second tree. The other leans quite precariously to the right, and it may be only a matter of time before it, too, falls.

I tend to get entangled in the plum tree's branches when I'm in the triangular area bordered by the low fence. So I don't go there often, and as a result weeds grow freely. This area is full of pebbles, so it isn't really suited for expanding my vegetable garden. To do so, I'd first want to sieve the pebbles out. But doing so requires being inside the plum tree triangle, and getting entangled... so I haven't (yet).

The wall in the background is new. The reason for its being new deserves a whole post of its own. The racist jerk builders (the neighbours picked them) who did the work left bits and pieces of rubble (pictured further below) in the area, which I collect as I find it.

A swimming pool is a pleasant luxury in the peak of summer, but I have strong doubts whether it's worthwhile having one. Maybe if one has kids (I don't).

A swimming pool eats chlorine. You get to choose in which form you feed it chlorine (okay, or maybe ozone), but unless you hulk out on a miniature wetland, you're going to have to keep it disinfected and clear of algae. I haven't optimized my pool, so it costs me about R400 per month in granular chlorine and electricity in the height of summer. (Far less - maybe half - in winter.)

Summer is also the season of almost daily topping up the unoptimized pool. On a hot windy day I can lose a centimeter or more.

The leaf skimmer has broken off the inlet port twice now, presumably when the aquabot bumped into it when the hose was long enough to let it reach. Pool parts are surprisingly expensive for what amounts to a small bit of PVC. I suspect there's just not enough competition in the local pool market to allow a significant consumer surplus.

Speaking of consumer surplus, the price of a new aquabot is more than I'm willing to pay. Mine broke: the lead ballast finally wore through the bottom part of the aquabot body, allowing the ballast to fall out, which left the aquabot as a whole buoyant. That caused it to float at the surface, doing its little chup-chup-chup dance with nothing but water and air to suck at.

Aside: does anyone know what aquabot body parts are made of? It seems too flexible to be PVC, but I don't know how to identify this plastic material reliably enough to know how to patch up the worn-through part.

With no aquabot in action, some leaves accumulate at the bottom. I sweep them around with the pool brush regularly, and eventually they find their way close enough to the weighed-down end of the pool hose that they get sucked into the weir basket. I'll have to fix the aquabot quite soon now, before summer, as this mitigatory practice will not suffice when the algae can photosynthesize with abandon.

One windy winter's night I heard a "swoosh" from outside. The following morning I discovered that that's the sound a bougainvillea makes when it falls over. I cut nearly all of it back, and I'm undecided whether I should keep it at all. The only reason I haven't yet removed it completely is that one stem still seems secure and healthy, and it's very pretty when it blossoms.

Unfortunately a bougainvillea has thorns, which makes it a nasty plant to work with. I simply left the cuttings where I dropped them, next to the deck, and simply burned them after a braai. That way I didn't have to fight the thorns to get the branches into the shredder - the opening to my fireplace is much, much larger (and hence more permissive) than the orifice leading to the shredder's blades.

Do you like ivy? Or Virginia creeper? I think they're pretty too, but my oh my are they aggressive plants. I decided rather to clear the back garden wall completely than to have to fight these plants season after season. They're tricky to get into the shredder though: the ivy because the bulk of the stems are so soft, and the creeper because it gets so entangled with itself.

When I moved in, the Virginia creeper had covered nearly the entire north wall of my house. It had attached itself to the eaves of the roof and from there had grown between the gutter and fascia boards, and even under the roof tiles into the roof space. Last year I decided to deracinate this creeper, so had to dig up the bed next to the wall and in places also had to lift paving blocks to get at the roots.

Now Nasturtiums have taken over where the creeper once was. If I'm ever in a fix for salad, I know where to go. The snails like it too, but Nasturtiums seem able to grow faster than snails can eat, so they're mostly whole.

Most wood doesn't do too well outdoors, and this arch is no exception. I'm guessing that it was untreated pine, simply painted. It has now rotted, and at some point the top of the arch broke, only to be held up by the then still present creeper stems. Now I counter the tension from the regular-looking washing line that hangs between the arch and the added-on bathroom, with a rope that doubles as replacement washing line. One of the formal lines, of course, broke, having been weakened by years of UV.

My advice? Live in an apartment if you don't crave a garden. It should be a craving, not just a vague desire for a manicured lawn and a few potted plants. Alternately, you could pave your garden, if that's your aesthetic. Or be wealthy, and hire gardeners.