Saturday, May 30, 2009

Apple #386: Light Bulbs

The night before I left for my trip, I was up very late (surprise, surprise), taking a shower. My shower is in the basement, and dimly lit. I had a clip-on desk lamp that I had clipped to the side of the shower so that the bulb would shine right into the shower and make it much easier for me to see.

So I was showering away, happy as a very tired clam, when all of a sudden there was a great POP! and I felt something sting my shoulder and heard a lot of glass tinkling. I realized the light bulb in my clip-on desk lamp had exploded, and the floor of the shower was full of broken light bulb glass. The stinging on my shoulder was from where one of the shards of light bulb glass had landed. It hadn't cut me; rather, because the glass was so hot, it burned my skin in a patch exactly the same shape as the piece of glass. Weeks later, I still have a mark there.

All that's left of the light bulb that exploded in my desk lamp.
(Photo by the Apple Lady)

Naturally, this got me thinking about light bulbs and what makes them explode.

Why Light Bulbs Explode
  • Lots of people say that poor-quality, off-brand light bulbs are prone to exploding. This is because the off-brand bulbs typically aren't sealed well, and they allow air to leak into the interior of the bulb, which can cause an explosion. Yeah, this was probably a cheap, grocery-store brand of bulb that I had in there.
  • If the light bulb is hot enough and even a few drops of cold water land on the bulb, the cold will make the glass contract unevenly in that spot where the cold water hit it, and the light bulb can explode. I don't think this was the cause in my situation because I was in the shower and the water was hot, not cold. But still, that's good to know.
  • If the light bulb had flickered before exploding (it did not), that could indicate a bad electrical connection -- faulty or old wiring, perhaps. That poor connection could send spikes of power to the bulb, causing it to overheat, which would in turn put too much stress on the glass, which would then explode.
  • It is also possible that as a light bulb gets old, the filament which is pretty much the glowing part that carries the light & heat from one pole to the next inside the bulb gets weaker and weaker. Most of the time, the filament snaps and the bulb just plain burns out. But sometimes the filament can sort of fly off at one end, or arc. If the arc keeps burning, a lot of heat and pressure can build up inside the bulb very quickly, which could cause the glass to shatter.
  • Power surges can also cause light bulbs to explode. Lots of people report power surges happening after lightning struck very close to the house, and the resulting surge of electricity blew out most or all of the light bulbs in the house. This slow-motion video shows a light bulb pulsing with what looks like some sort of power surge before it explodes.

(Diagram of a light bulb from

How the Explosion Happens

What is actually happening during the explosion is a nice bit of science fun.

  • The inside of the bulb is filled with inert gases like argon and nitrogen. Those gases displace the regular old air that you and I breathe, and the light bulb is sealed up with those gases in there. That creates a vacuum.
  • The reason you want a vacuum inside a light bulb is because the filament will burn longer. If you've got too much oxygen (which is present in regular air) the filament, no matter what material it's made of, will burn up in a couple of seconds. If you remove all the oxygen, the filament won't burn at all. The inert gases allow the filament to glow instead of burning. In addition, the gases carry the radiant heat out to the glass, which makes the entire interior of the bulb glow.
  • Another important piece of light bulb construction is the strength of the glass. Once the inside of the bulb is a vacuum, the pressure of the outside air is going to push in against the glass. If the glass is weak and cheap, it'll break and the outside air will rush in. Better-made bulbs use thicker glass that better withstands pressure from the outside air.
  • Regardless of the thickness of the glass, it is still possible that the glass might develop a crack, or a leak somewhere. If that happens, air from the outside of the bulb rushes in. Since the outside air is at a higher pressure than the no air inside the bulb, that new air pushes against the glass from the inside out, resulting in an explosion.
This video explains how a light bulb works and shows in super slow motion how the tungsten filament begins to glow. At the end of it, the light bulb explodes. From the way the filament is dangling, I would guess that explosion was caused by the filament arcing.

Light Bulbs with Mercury Inside 'em

What I've been describing are incandescent light bulbs. These types of bulbs, after more than 100 years of service, have been declared to be energy-inefficient. The European Commission have decided that by 2012, incandescent light bulbs will are to be permanently phased out. Australia is giving incandescents the boot by 2010. Lots of people who care about the environment have said that incandescent bulbs are not energy-efficient enough and that we should find alternatives.

So people have been developing alternatives. One type that you've probably heard about is known as the Compact Fluorescent Lamps, or CFLs. The US Energy Star program, for one, says of these bulbs

If every American home replaced just one standard incandescent light bulb with a long-lasting CFL, the resultant energy savings would eliminate greenhouse gases equal to the emissions of 800,000 cars.

So I've bought into this. I have three CFLs currently in use in various fixtures around my house. The oldest one I put into the fixture four years ago. Still working just fine.

However, these bulbs contain mercury. After my cheap-o incandescent bulb exploded all over my shower, I thought, what if that had been one of those CFLs with mercury inside it?

A compact fluorescent lamp (CFL) with the Hg symbol on it. As you probably remember from your high school chemistry class, Hg is the chemical symbol for mercury.
(Photo from

  • Remember when everybody got all concerned about the mercury in thermometers, and we all had to throw out our old, bad, dangerous mercury thermometers and get new ones?
  • Just in case you don't remember or never heard, mercury is a neurotoxin. That means it damages nerve cells, and in a hurry. Even in very small amounts, it can cause brain damage, seizures, and even kill you.

What a blob of mercury looks like when it's on the loose, so to speak. Really cool, and really dangerous. There is only a tiny percentage of this amount of mercury -- in powder form -- in CFL bulbs.
(GNFDL photo sourced from World News Network)

  • So, if we were so concerned about mercury in our thermometers, why aren't we concerned about it now, when it's in our light bulbs?
  • People argue that the amount of mercury contained in these bulbs has less of an impact than the incandescent bulbs. They say this because the electricity which feeds into the light bulbs is still predominantly made by coal-fired power plants, which send a heck of a lot more mercury in gaseous form into the air than the CFL bulbs have inside them.
  • In fact, there's a ton -- literally -- more mercury coming out of power plants than there is coming out of light bulbs. In 1999, the EPA estimated that coal-fired power plants were emitting an average of 48 tons of vaporized mercury into the air per year. That's more than all other sources of human-made mercury combined.
  • Yet another reason the mercury produced by power plants is more of a concern than the mercury in a light bulb is because mercury in the air is far more toxic than a blob of mercury on the floor. Once it can gets into your lungs, you will breathe it over and over.
  • But those power plants aren't shooting the mercury directly into your house, right? (Or at least, you like to think they aren't.) Whereas those light bulbs with the mercury in them, those are in your house.
  • So let's say you have one of the CFL light bulbs, and you drop it, and it breaks. Out comes the mercury powder. That's only about 4-5 milligrams of mercury. (Those old thermometers, by the way, had 500 milligram blobs of mercury in them.)
  • Sure, it's just a little bit of powder, but it's still mercury. If you touch it, the mercury will be absorbed through your skin. If you eat it, that's even worse. Nerve damage, seizures, death. We get it. This is why the packaging on most CFL bulbs has explicit instructions about not touching the mercury or not eating it.

Those recommendations, in brief are:
  1. Open the windows immediately to less the concentration of vaporizing mercury in the air in your home.
  2. Turn off the air conditioning or forced-air furnace.
  3. Do not touch the spilled mercury.
  4. Do not use a vacuum cleaner to clean up the glass or the mercury, as it will vaporize the mercury faster.
  5. Clean up the broken glass carefully and immediately, while wearing gloves and sliding the broken glass and mercury powder onto something disposable yet sturdy like stiff cardboard.
  6. Use sticky tape to remove any leftover fragments or mercury.
  7. Wipe the place where the spill happened with clean, damp paper towels.
  8. Place all the paper towel and gloves and whatever else you used to clean up the mercury into a sealed plastic bag, and take the bag to your friendly, local Household Hazardous Waste collection site.
  9. (More detailed tips on cleaning up a broken CFL bulb are available here and still more details are available here)

Of course some enterprising individuals are capitalizing on this problem. Which is not necessarily bad. One company sells this CFL clean-up kit, which includes things like thick gloves, shoe covers, face masks, and wet wipes, for $9.99.
(Photo from CFL Clean-Up Kits)

  • But let's pretend that you neither touch nor eat the mercury powder. Let's pretend that the mercury powder sits there on the kitchen floor for a while. Eventually, it will evaporate.
  • Even if that mercury powder on your kitchen floor evaporated instantly, creating the highest possible concentration of vaporized mercury in your home, that concentration would amount to only 0.2mg/m^3. In layperson's terms, that is less than half of the level at which OSHA thinks mercury in the air is dangerous. Furthermore, that airborne mercury will dissipate over time, further reducing the concentration of mercury and the amount that could get into your lungs.
  • You can wash your clothes, you can air out your house, you can toss out the paper towels you used to wipe up the spill. But it's a lot harder to get rid of those 48 tons per year of mercury that are floating around in the air and getting into the water and the fish and the soil.
  • So people say that CFLs have such a comparatively small concentration of mercury in them, that amount of mercury is manageable, and thanks to Home Depot's help, you can safely dispose of the bulbs. And the amount of energy you'll save by using CFLs will actually reduce the amount of mercury being pumped out by those power plants.

Still, I'm going to put out a call for research. Very officially. Hey, scientists, will you figure out what to do with the mercury? And find something less toxic to put in these light bulbs? Thanks.

For more information on how to handle a mercury spill, and links to places where you can recycle your CFL bulbs, check out the EPA's site on mercury spills.

The Dollar Stretcher, Exploding Light Bulbs
MSN Q&A, Could a power surge really make a light bulb explode (cached), Light bulb exploding, May 2, 2009
InterNACHI, Exploding light bulbs
"Experts doubt exploding light bulb theory,"
St. Petersburg Times, April 7, 2005
Sawaal, Why does an electric bulb explode when it is broken? September 4, 2007
WikiAnswers, How does a light bulb work?
Howstuffworks, Light bulbs: The filament
Theodore Gray, You Do It: Make Your Own Light Bulb, (not for beginners) Live Science, February 27, 2006
James Kanter, Kissing Edison's Light Bulb Goodbye,
The New York Times, March 18, 2009
Elizabeth Shogren, "CFL Bulbs Have One Hitch: Toxic Mercury," NPR, All Things Considered, February 15, 2007
Northeast States for Coordinated Air Use Management, Mercury Emissions from Coal-Fired Power Plants: The Case for Regulatory Action, (PDF) October 2003
Helen Suh MacIntosh, Is Mercury from a Broken CFL Dangerous? Ask TreeHugger, May 1, 2007, Energy-saving bulbs (CFLs) release dangerous amounts of mercury when broken
Julia Layton, Is the amount of mercury in CFLs more dangerous than the amount in fish? Howstuffworks


  1. Great post but I have a question about one of life's great mysteries: If I put 4 light bulbs in my bathroom fixture (all from the same box, purchased at the same time, and all brand new) why will one burn out before the others? So annoying!

  2. Good question, Pam. I'm going to guess at the answer. Since I'm guessing, I've got some philosophizing to throw into it, too.

    Most manufactured products are made to meet a range of acceptable performance. For light bulbs, the performance goal would be hours of operation, or life expectancy. Usually on the package it says something like, "about 900 hours." But it will never say that the bulbs will definitely last a specific amount of time.

    The reason manufacturers have such ranges is because even though a product might be made on an assembly line from start to finish, different things can happen from one batch to the next. It's possible that one batch of light bulbs, made on the exact same set of equipment, but operated even a few minutes later than when the previous batch was processed would result in light bulbs that meet a slightly different range of performance.

    Perhaps more importantly, all such manufactured things are made by humans. Even if a human person never even touches the thing but it's entirely made by machine, these things are still made according to human design. As we know, humans are faulty. They don't get everything right every single time.

    The next time you're baking cookies, for example, try making one cookie exactly the same as any other one you've already put on the cookie sheet. Or the next time you brush your teeth, try to brush them in exactly the same way you did the last time. You see my point. It's really hard to do anything exactly the same way twice.

    Now try doing that with light bulbs, four times over.

  3. I'd guess that it was water drops that made your bulb explode. It's cold water, relative to the temperature of the glass on the bulb, not relative to what us humans find cold.


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