IMPORTANT NOTE CONCERNING COMMENTS: For a while, I tried to keep up with individual responses to comments on this article, but I don’t have the time to do that with my current schedule.  Many of the new comments pose scenarios that have already been addressed in previous questions or in the article; if you have a question, please take a look through previous questions and responses.  If I do respond, it may take a couple weeks or more, so again I suggest looking through previous responses.


Quick quiz: rank the following three situations in terms of the danger they pose to your child.

  1. You find your child playing outside with a drop of liquid mercury from a broken thermometer.  He’s been handling it for a half hour, and you think he even ate some of it.
  2. Your child accidentally knocks over a lamp with a compact fluorescent bulb in it, and the bulb breaks, though the shards end up out of the way so your child can’t be cut from them.
  3. Your child loves fish, and he wants to have tunafish sandwiches for the fifth night this week for dinner.  You give in.

If you answered (1) as the most dangerous, you need to read on.  Situation (1) is actually by far the least dangerous of the three situations in terms of mercury poisoning.  In this post, I’ll separate out myths from facts concerning liquid mercury (which isn’t very dangerous at all) versus the real dangers of mercury poisoning.

One day, a few years back, I was performing a typical simple lab experiment in an undergraduate chemistry laboratory.  As usual, we were using thermometers, and they each contained a small amount (a few grams) of mercury.  The student working next to me broke her thermometer.  I went to tell the lab assistant.  He immediately freaked out.

First he moved my neighbor away from her station.  Next he found a giant container of laboratory-grade sulfur and started literally dumping it all over the floor.  A few minutes later, he asked me to move, and he started wondering aloud whether masks were necessary.

I moved my experimental setup (no small task, since this was a complicated distillation experiment), but at the end of the lab period that day, I went to talk to the lab assistant about what happened.  I politely showed him the safety sheets for each of the chemicals we were using during the lab (including some rather nasty organic solvents and solutions).  Mercury was actually one of the least dangerous chemicals we were using that day.

Some months later I talked about the experience with my father, who had worked in a mercury room in a plant that refined and distilled mercury back in the 1960s.  He had worked there for a number of years, and the safety regulations were much more lax than we’d see today.   They handled hundreds of buckets of mercury every day, splashing around quite a bit.

With all of this mercury around them, you’d think that these workers would have a very high rate of mercury poisoning.  But they didn’t.  They only introduced significant safety measures after one worker displayed symptoms, and he had been working there for years and was notorious for being lax with things like hygiene.  Within a month or two of the new regulations (which were still quite lax compared to today’s standards), this guy’s symptoms cleared up, and he lived to a ripe old age with no apparent long-term effects.  My father kept up with some of his coworkers over the years, and they haven’t appeared to suffer greater than the normal population from long-term effects.

I don’t claim that my father’s experience should be viewed as conclusive, but it does suggest that mercury exposure is often not as deadly as many people believe.  That said, there are some circumstances where we need to be concerned about mercury exposure, as I will discuss below.  Surprisingly, lobbyists have managed to eliminate sources of mercury that were not that dangerous to humans while introducing new sources recently that carry greater risks.

Elemental mercury versus mercury compounds

Many people have heard the story about a scientist who spilled a couple drops of mercury on a gloved hand back in the 1990s and died of mercury poisoning only a few months later.  I myself was told about this story by the lab director in my undergraduate chemical safety session.

The scientist in question was Karen Wetterhahn, and she was an experienced chemist and expert in toxic metals, who took reasonable safety precautions, though she chose to wear common latex gloves rather than heavier plastic ones, probably because the thin latex gave her better control and dexterity for handling the lab equipment.

While this story is certainly tragic, what most people do not know is that the chemical she spilled was not simple pure elemental mercury, but a compound called dimethylmercury.  As anyone who has taken basic chemistry knows, compounds often have very different properties from their constituent elements.  Table salt (sodium chloride), for example, is made up of a highly flammable and potentially explosive metal (sodium) and a deadly gas (chlorine).  But the resulting compound is perfectly safe in reasonable quantities and tastes really good on french fries.  In fact, our bodies need a certain amount of salt to function; without it, we’d die.  (Keep that in mind the next time you hear about a city wanting to ban salt — the problem is overconsumption of heavily salted processed foods, not salt itself.)

Dimethylmercury is one of the deadliest mercury compounds.  Its particular form makes it easy for it to permeate the skin and get into the bloodstream, where the mercury can easily be deposited in the brain, kidneys, and other organs, resulting in acute mercury poisoning.  Even in very small doses, mercury can be quickly absorbed — from a few drops, Prof. Wetterhahn experienced a blood mercury level of 4 mg/L, about 80 times higher than the threshold for mercury poisoning (0.05 mg/L).

Fortunately, dimethylmercury has few applications outside of chemical laboratories, so it is very unlikely that any member of the general population would ever be exposed to it.  Using this unfortunate story of a Dartmouth professor to scare people about mercury would be like using an anecdote about a sodium metal explosion in a lab to scare people about table salt.

Elemental mercury is actually much more difficult for the body to absorb than methylmercury compounds.  While dimethylmercury does not occur naturally, other methylmercury compounds can be found in natural sources, particularly large fish, who accumulate the mercury by eating smaller fish and other life forms in water that contains mercury.  When we eat a fish who has high levels of methylmercury, which is soluble in fat, most of the mercury in the fish can be absorbed into our bloodstream (usually over 90%), while if we were to eat elemental mercury itself, very little of it would be taken up by our bodies — 99.99% of it would come immediately out in our waste.

This is important to note — methylmercury compounds are about a thousand times more dangerous than elemental mercury.  Thankfully, we only tend to encounter them in relatively small doses, particularly in fish, though smaller amounts of mercury end up in soil and therefore are ingested in other meats, as well as grains, fruits, and vegetables.

Mercury is almost everywhere in the environment.  In parts per million (ppm), state/EPA standards for mercury in safe drinking water are ≤ 0.001-0.002 ppm, while typical plant/animal sources fall anywhere between 0.0001 and 0.01 ppm.  Maximum values are around: fruit ≤ 0.04, vegetables and grains ≤ 0.02, meat ≤ 0.05, rice ≤ 0.2 ppm.   All fish tend to average ≈ 0.05,  while tuna ≈ 0.2 ppm.  Some large fish may have values in excess of 0.5 ppm.

(Keep these numbers in mind when you read about, for example, the high fructose corn syrup mercury scare, which found levels of mercury around 0.00003 to 0.00035 ppm in goods produced with high fructose corn syrup.  These levels are small enough that the mercury could have come from any number of sources in the food manufacturing process.  I’m not a fan of eating a lot of processed foods, but in this case, HFCS isn’t clearly the culprit for the tiny amounts of mercury in some foods.)

So far, we know that some mercury exposure is inevitable, that elemental mercury isn’t anywhere near as dangerous as methylmercury compounds, but clearly mercury sometimes is dangerous or even deadly.  When should we be concerned?

When not to worry

Despite the ban on many household mercury items like thermometers and barometers in many European countries, these items are not generally a cause for concern, at least in terms of human health.  The greater concern for these items is environmental, since mercury-containing items should never be simply thrown in the trash.  The true hazard from thermometers and barometers is the accumulation of mercury from millions of these things being thrown into landfills, which eventually contaminates drinking water, soil, and fish.  Nevertheless, this source of mercury has always been quite small compared to industrial sources, these days particularly coal-fired power plants.  (Most coal contains mercury, which evaporates into the atmosphere as the coal is burned.)

“But wait,” you say, “I thought a broken mercury thermometer was a real health hazard.”

Well, in most circumstances, no.

Elemental mercury is only dangerous in vapor form.  Exposure to a little liquid is unlikely to cause any harm, let alone major health effects.  Many older people remember playing around with mercury from broken thermometers when they were kids; they haven’t all died.

In fact, it’s quite unlikely to get mercury poisoning from playing around with mercury in your hands or even eating it.  As mentioned above, 99.99% of ingested elemental liquid mercury will be excreted right away.  Studies on other animals have shown that it takes a relatively large amount of ingested mercury to cause any harm.  A normal adult would have to eat at least a quarter pound of mercury to approach a minimum level for lethal exposure!  (I don’t encourage you to try this, but it’s important to note that small doses of mercury have been ingested for health benefits for centuries; few would have continued the practice if a small dose was likely to kill you.)

Similarly, elemental mercury is absorbed very slowly through skin; mercury vapor is absorbed about 50 times as fast through vapor in the lungs than through the skin, so unless you’re breathing through a sealed mask while bathing in pure mercury (which would be difficult, because you’d float), skin exposure is very unlikely to be significant.

Again, I’m not saying it’s a good idea to go out and play with mercury with your bare hands every day, but unless you have cuts and bruises (where the mercury could more easily permeate your skin), it’s really unlikely to cause harm to do it once in a while.  It’s just as likely that chronic exposure to other household chemicals on your skin will result in health effects.

What about vapor?

However, elemental mercury is not a safe substance.  As already mentioned, mercury vapor is the most easily absorbed form.  The vapor pressure of mercury is relatively low, which (in layman’s terms) means that if you set out a bunch of liquid mercury in an enclosed space, enough of it will eventually evaporate to lead to dangerous levels of vapor.

This leads to the first rule of mercury safety: Don’t expose mercury in an enclosed space.  Be sure to ventilate any area well during and after mercury is exposed.

How much ventilation is required?  It depends on how much mercury is exposed, what form it’s in, and how long it’s exposed.  The volatility of mercury at room temperature (68 degrees F) is 0.056 mg/hr-cm², which means 0.056 milligrams of mercury vapor will evaporate per hour for every square centimeter of surface area.  Suppose you have one medium drop of mercury sitting out with an area of 1 square centimeter (a little less than half the size of a dime), which would be roughly the size of a drop from all the mercury in two or three medical thermometers (or one lab thermometer).  About 0.056 mg of mercury would evaporate from that drop within an hour at room temperature.

Is that a dangerous quantity of mercury vapor?  Probably not.  As mentioned above, the blood-mercury level for minimum toxicity is considered to be 0.05 mg/L.  A typical adult has about 5 liters of blood, meaning it would take a minimum of 0.25 mg of mercury to lead to basic poisoning levels.  If the space was well-sealed (not ventilated at all) and you stood over that drop of mercury and breathed deeply to absorb all the fumes for an hour, you’d take at most 0.045 mg into your body.  (Only about 80% of mercury vapor breathed in is actually absorbed.)  It would take quite a few hours of intensely breathing in mercury fumes above a drop of mercury before you could even approach poisoning levels, and that is in a closed room.  If you ventilate the space even minimally, it’s very unlikely that a small amount of mercury like this will lead to poisoning through vapor.

But don’t take my word on it.  Someone did an actual peer-reviewed study where they put two drops of mercury — one a bit smaller and one a bit larger than the one I assume here — in an office and let them sit uncovered for a few months.  The office wasn’t ventilated in any special way; it was just a typical office.  The rate of evaporation was measured frequently by weighing the amount of mercury that remained in each drop, and the mercury levels in the air around each drop were measured with a meter.  At no point did the mercury levels even approach dangerous levels; in fact, they were always well over a hundred times lower than the OSHA limit for toxic mercury exposure in the workplace.  The study estimated that it would take 500 to 2000 such drops from broken thermometers, all sitting around in a small office, to exceed the limit for toxic mercury exposure from the vapor given off.

Of course, children have a smaller amount of blood, which means that mercury they breathe in will be more concentrated.  But again, unless they are deliberately inhaling the vapor in an enclosed space, a small amount of mercury is unlikely to cause significant harm.  If you find your child playing with a tiny bead of mercury from a broken thermometer outside, you probably have no need to worry.  Just tell them to wash their hands, and unless they spilled it on their clothes, that’s all they need to do.  Even if they ate some on a dare, there’s usually no cause for alarm.

Rule #2 of mercury safety: In enclosed spaces, keep mercury exposure times short.  Avoid long exposure times or frequent exposure unless you have good ventilation.

Playing with a large quantity of mercury in a small enclosed space, of course, is a different story.  But even larger quantities of mercury in bulk are unlikely to emit enough vapor to accumulate if the space is ventilated properly.

Note that I say in bulk.  That is important, because so far we’ve been considering mercury in a single drop.  When you split mercury up into smaller drops or tiny microscopic droplets, the surface area increases.  Remember that the volatility depends on the amount of time the mercury is exposed and its surface area.

A large drop or even a large puddle of mercury (even a pound or more) is unlikely to be a health hazard in a minimally ventilated space.  But if you break up mercury into smaller drops, the surface area increases greatly, leading to much more rapid evaporation.  Having tiny drops instead of one large drop could increase the evaporation rate by tenfold or more.  Tiny droplets could increase it to 25 times the evaporation rate of a single large drop.  Instead of taking many hours or days for vapor levels to reach a dangerous level in an enclosed space, a mist of tiny mercury droplets could evaporate to poisonous levels in less than an hour, or even within minutes if the quantities are large enough.

Rule #3 of mercury safety: Large drops or puddles of mercury aren’t usually a problem.  Lots of tiny drops are more of a problem.  A mist of tiny droplets is almost always a potential hazard.

If you spill the mercury

The danger of small drops leads to a discussion of safe handling procedures. First of all, if you are actually handling bulk mercury deliberately (playing with it, doing experiments, etc.), be careful.  Work on a hard surface with a non-porous container for the mercury.  Always keep the mercury above a non-porous container or surface.  If you accidentally drop a little onto your work surface, gather it back up with an eye dropper, and either return it to the bulk mercury (if clean) or set aside for cleaning (if dirty).

Rule #4 of mercury safety: Always work on a non-porous surface and containers to avoid difficult cleanup.

Of course, mercury is sometimes accidentally spilled off your work bench.  What to do then?

If mercury is spilled outdoors, it’s not good for the environment, but the vapors are unlikely to accumulate because of the constant air motion.  Clean up what you can (see below for instructions), put in a sealed bag, and dispose properly.

But indoors, mercury spills should be cleaned up immediately.  The process is simple if the mercury is in bulk form (like drops from a thermometer) and it spilled on a hard surface, and it doesn’t require special equipment.

Get a few items:

  • Disposable glass jar with a good sealing lid
  • A couple sheets of stiff paper, like index cards (or razor blades, if you prefer)
  • Eyedropper (useful if there are small beads in cracks or corners)
  • Duct tape
  • Flashlight

If there is glass from a thermometer or something, clean up large pieces of that first.  Then use the paper or razor blades to push the mercury drops together and carefully deposit the mercury into the container.  You can also use the eyedropper to pick up the beads instead or to get to beads that are tough to get to with the paper.  Use the flashlight to shine around the area; mercury is highly reflective, so you should be able to see any remaining beads.  When finished, seal the jar, put tape around the lid to seal it further, label it as “hazardous mercury waste,” and take it for mercury disposal.  (Many communities are now dealing with mercury disposal for CFLs — see below — you can try there.)

If the spill is on a hard surface, that’s it.  It should be easy to clean up.  You may want to ventilate the area as well, particularly if the amount of mercury was large or if it broke up into a lot of small drops before cleanup.

But don’t delay in doing this.  If you leave spilled mercury around on the floor, it will be stepped on, which will break up the mercury into smaller and smaller drops, which (as mentioned) will evaporate more quickly.  It could also contaminate shoes and clothing.

If the spill is on a soft surface, like a rug or a couch, you’ve got a bigger problem.  Mercury on clothing is also an issue.  In that case, begin by following the instructions above, being careful not to push mercury into the soft material so that it will break up and be absorbed.  After getting all the large bits, use duct tape (or another form of very sticky tape) to collect mercury from the surface, as you would a lint brush.  Hopefully, if the spill was small, and you cleaned it up before grinding it into the soft surface, you’ll get most of the mercury off.

At this point, you have to consider whether you want to get rid of the contaminated item (rug, couch, clothing, etc.).  If you don’t have children or infants playing around the item, and you cleaned up the spill promptly, there’s probably no need to worry.  Just ventilate the item well for a few days, and be sure to ventilate properly when vacuuming or cleaning the item for the first few times after the spill.  Don’t vacuum or otherwise clean the item before the spilled mercury has been removed as much as possible. (In fact, I’d avoid vacuuming the area altogether for a few months, rather than risk contaminating the vacuum cleaner.)

If you have children, and you can’t throw the item away, put it in a well-ventilated spot for a few weeks or months for the mercury vapor to dissipate (the rate of dissipation depends on temperature).  If you’re really concerned, have a professional check the area with a meter to check for mercury vapor before returning the item to use.

Again, if the mercury was spilled in bulk (beads or drops) from a broken thermometer, old mercury switch, etc., and is cleaned up before it is ground into fine drops on a soft surface, significant contamination is unlikely.

Rule number 5 of mercury safety: Clean up spills promptly.  Hard non-porous surfaces are rarely a concern.  Soft surfaces may need more attention, and may need to be disposed of or ventilated if small droplets are absorbed into the material.

When to worry

Occasionally, we hear stories in the media about mercury poisoning.  Rarely does such poisoning result from infrequent exposure to elemental mercury in bulk.  Liquid mercury is nothing to be afraid of, as long as it is handled with care — in a well-ventilated space always on and above non-porous materials.  If handled with a minimal amount of care, it’s just as safe (and actually safer) than the vast majority of chemicals chemists deal with on a regular basis.  You’re probably more likely to suffer ill effects from harsh household cleaning products used in enclosed spaces.

But when do poisonings occur?  Well, it’s unusual, but they can occur in children or infants who play on soft contaminated surfaces, which is why I recommended prompt and careful cleanup above.  If mercury is ground up into small droplets by people touching or walking on a contaminated surface, they can get on clothes and shoes and be carried around to contaminate a much larger area.

Other poisonings occur when using lots of mercury in enclosed spaces for a long time.  Again, these are rare and violate the principles outlined so far.

One further type of incident occurs when mercury is heated.  Recall that the volatility number I gave above was for room temperature.  That number increases with temperature, and if the mercury is actually heated even a bit, it will being to evaporate much more rapidly.  Incidents have been noted where poisoning has occurred where a mercury spill was cleaned up with a dustbin that was subsequently left on a hot stove.  Again, this is rare and required multiple violations of the safety guidelines already noted.

But it is a reason to keep mercury thermometers out of ovens.  Breaking a mercury thermometer in an oven is a major issue — if it can’t be easily cleaned up, you may need to buy a new oven.  Breaking a thermometer on top of a stove or elsewhere in a kitchen is usually less of an issue, since it’s unlikely that the mercury will get into a little crevice that can’t be cleaned up.

Rule number 6 of mercury safety: Never heat mercury unless you’re a professional chemist who knows what you are doing and are following proper safety procedures.  Keep loose mercury away from heat sources, and if it spills on or inside one in a way that you are not certain it can be cleaned up completely, consult a professional to check levels.

Up and coming mercury safety problems

There are two final problem with mercury that are currently emerging.  Be aware of these problems, because they represent the newest and most dangerous threats for mercury exposure.

The first is rising levels of mercury in large fish.  Most people know this already, but pregnant women and small children should limit their consumption of large fish.  Canned tuna is most notorious for this, but swordfish, shark, and other large fish are also a concern.  Of course, fish is known for its fatty acids which aid in brain development and have other health benefits, so it shouldn’t be avoided completely.

Nevertheless, these levels are not to be taken lightly.  Parents who would completely freak out if they saw their children playing with mercury from a broken thermometer happily give their children tuna sandwiches to eat.  This is preposterous.  Canned tuna (as mentioned above) has about 0.2 ppm of mercury, in the methylmercury form that can be easily absorbed into the bloodstream.  That means that a 150 gram can of tuna has about 0.030 mg of mercury that will be absorbed into the body.

Recall that someone playing with a drop of mercury in an enclosed space and heavily trying to breathe in vapors for an hour would at a maximum absorb about 0.045 mg of mercury.  A child playing with liquid mercury for an hour even in an enclosed space is like giving him a can of tuna to eat; in fact, since it’s unlikely that the child would breathe in all the vapor, the can of tuna is probably quite a bit worse.  If the tuna is solid white albacore (about 0.35 ppm of mercury), the tuna is definitely worse than playing with liquid mercury.

I don’t mean to make it sound like children shouldn’t eat tuna.  (Well, I might argue that no one should eat tuna right now in any case, just to avoid depleting overfished waters, but that’s an argument for a different post.)  On the contrary, both of these things — eating tuna or playing with liquid mercury — are unlikely to cause problems unless done very frequently.  Unless your child is eating more than a can of tuna per week — or playing with mercury for more than an hour per week — poisoning is unlikely.

Rule number 7 of mercury safety: Pregnant women and small children should limit consumption of large fish to avoid complications from mercury poisoning in large doses.  However, the health benefits of fish outweigh these concerns as long as servings are limited to once or twice per week.

Comparing playing with mercury to eating tuna may seem odd, and I don’t recommend that children play with mercury, because it is a hazardous substance.  But there is no reason why older children can’t be shown it once or twice to demonstrate its admittedly very cool properties, and they can even touch it or hold it with proper precautions and supervision.

The second (and in my mind, more distressing) development is the rapid worldwide adoption of compact fluorescent lights (CFLs).  I have more to say about them in a future post, but for now I just consider the home safety ramifications.

As was discussed in detail above with calculations, the chance of a drop of mercury from a broken thermometer resulting in poisoning is quite low, as long as it is cleaned up.  Large drops are not a problem.

CFLs contain only a small amount of mercury, typically 3-5 mg.  (Some low mercury bulbs contain 1 mg or less, others contain 10-20 mg.)  However, when a CFL breaks, that mercury is released in a fine mist of tiny mercury droplets, the most dangerous form.  While I’d say it’s safe for a child to play with a small drop of mercury for a few minutes, if I broke a CFL, I’d get the child out of the area immediately.  I’d evacuate pregnant women and infants from the entire house temporarily.  Actual studies have shown that a hazardous amount of mercury can get airborne within minutes, again because of the small drops.  Children or infants playing in the area are particularly vulnerable, not least because they are closer to the floor where the vapor is likely to be most concentrated.  Pregnant women or infants should not be allowed in the room or assist in the cleanup.  Unlike adults, developing brains appear to be more sensitive to high acute mercury exposure, so a sudden mercury spike could lead to permanent damage.  A single CFL break is unlikely to result in symptoms of acute mercury poisoning (though breaking a whole bunch of fluorescent bulbs in a small enclosed space could); nevertheless, it is better to be safe with small children.

For a broken CFL, close off the area immediately.  Evacuate children to a safe area.  Ventilate the room to the outside, and wait 15 minutes while the room ventilates and the initial mercury cloud dissipates.  Shut off heat, air conditioning, and fans that could potentially blow the mercury vapor around.  Follow procedures for cleanup outlined above.  If the break is on a soft surface (rug, sofa, etc.), I’d consider throwing it away if children play on it.  At a minimum, it needs to be removed to a well-ventilated area where children don’t play in order for the mercury to evaporate over a couple months.

If you think I’m being alarmist after saying that I’m okay with a kid rolling mercury around in his hands, I’m not.  The dangers of mercury are dependent upon the amount, its form, and the length of exposure.  A 1000 mg drop of mercury from a broken thermometer is unlikely to produce enough vapor to be concerned in a short time, but thousands or millions of tiny droplets from 5 mg from a broken CFL could cause poisonous levels to develop immediately and even if initially ventilated, the residual mercury can raise levels close to a contaminated surface to potentially dangerous levels within an hour or definitely within a day if unventilated.  These levels can remain high near the surface of a rug for weeks or months — high enough for children and particularly infants to be at risk.  It’s still not a very high amount, but it’s at least as high as mercury exposure from a large drop of mercury, except children playing in this area will be continuously exposed.  Unless the surface is continuously ventilated with moving air (unlikely in a home), I wouldn’t risk my child playing on it.

But don’t take my word on it, the Maine Department of Environmental Protection did a comprehensive study on it, including breaking bulbs in a number of different conditions and trying many different methods for cleanup.  You can draw your own conclusions from their large amount of data and experiments.  The first 15 minutes after a CFL break can have mercury concentrations in the room over 100 times the safe limit in the air, which is roughly the toxic exposure level in OSHA regulations mentioned above.  Moreover, concentrations near the surface of a rug can be well above the safe limit for weeks after the break, particularly when the initial cleanup is not handled properly and the air is agitated (for example, by a child playing there, a vacuum, etc.).

In clear terms — if you let your infant play on the carpet above a breakage site, it’s approximately equivalent to letting your infant stand a few inches away from a large drop of mercury continuously and breathe in the vapors.  Would you do the latter?  If not, don’t let infants or small children play in the area contaminated by a broken CFL.  For some carpet samples in the Maine DEP study, mercury levels stayed above the safe level for months.  Cleaning and vacuuming did not necessarily help, and in some cases seemed to make the situation worse by spreading the tiny droplet around to a wider area.  While I might let an older child play with a bead of liquid mercury in a well-ventilated space for a few minutes, I certainly wouldn’t let an infant breathe in vapors at the same or greater level for weeks or months during playtime.

Rule number 8 of mercury safety: Be very careful not to break CFLs.  Don’t expose children to the area of a broken CFL.  Consider not using CFLs in areas where children play or are likely to break them.

All of that said, I don’t want anyone to get the impression that CFLs are particularly dangerous.  They only should be a significant concern if broken in a porous area where small children or pregnant women are active — in that case, the concerns are the same as with any mercury spill, from a thermometer or otherwise.  Between CFLs and fish consumption, fish is by far the greater hazard.  But broken CFLs are not benign.  I only make them out to be a greater risk than thermometers because they immediately disperse their mercury in a way that is more dangerous and harder to clean up, and people may be less likely to think about the mercury content.  A broken thermometer on a carpet that isn’t properly cleaned up is just as much of a hazard, and probably a greater one.  However, mercury thermometers have become rather rare, while the expanding market for CFLs makes them a new risk.  Where possible, I’d recommend looking for brands with low mercury content or — even better — buy LED lights instead.

Finally, note that mercury does get excreted from the body.  Most of it will have a half-life of at most 60 days, so if a person does get toxic levels, within 60 days they will likely drop to half of that, and with another 60 days, a quarter of that, and so forth.  Studies have shown that chronic exposure of mercury does lead to deposits in the brain that can stay there for years, but the data on short-term exposure is less conclusive.

Nevertheless, if you are exposed to some mercury vapor, chances are that you’ll suffer no ill effects unless the concentration was very high or if you are exposed frequently for a long period.  You’ll excrete most of it from your body over the next few weeks or months.

There’s a lot of misinformation about mercury out there.  It is a hazardous substance and should be handled carefully.  But, except in certain unusual conditions (getting more common unfortunately with the rise of mercury in fish and the introduction of CFLs), mercury is rarely dangerous and is unlikely to cause poisoning.  Unfortunately, we have governments who take very inconsistent stances toward this hazardous material, such as banning items (mercury thermometers and barometers) which are unlikely to cause health problems except in unusual circumstances while forcing us to use products (CFLs, since incandescents have been banned in some places) that introduce significant new dangers.


To review the rules one more time:

  1. Be sure to ventilate any area well during and after mercury is exposed.
  2. Avoid long exposure times or frequent exposure unless you have ventilation.
  3. The smaller the drops, the bigger the danger; big puddles or drops are not generally hazardous.
  4. Always keep mercury above hard non-porous surfaces.
  5. Clean up spills promptly and properly.
  6. Never heat mercury unless you really know what you’re doing.
  7. Limit consumption of large fish, especially in pregnant women and children.
  8. Avoid breaking CFLs, and keep children away from them.


A good general review of issues in mercury toxicity can be found here.  If there isn’t enough information, you can find more information in the references cited in that article.


UPDATE (in response to many comments): One last piece of general advice I can give — a mercury thermometer break or a CFL break is highly unlikely to cause acute mercury poisoning, particularly in adults.  The amount of mercury is simply too small.  Children are probably only at risk if the break occurred in a play area and is not cleaned up, leading to long-term exposure.

It’s a good idea to take extra precautions, especially around pregnant women and small children, but the first thing you should know is that the danger from these types of mercury exposure is probably minimal except under unusual circumstances.

IMPORTANT NOTE CONCERNING COMMENTS: For a while, I tried to keep up with individual responses to comments on this article, but I don’t have the time to do that with my current schedule.  Many of the new comments pose scenarios that have already been addressed in previous questions or in the article; if you have a question, please take a look through previous questions and responses.  If I do respond, it may take a couple weeks or more, so again I suggest looking through previous responses.