Under Pressure: Attack of the Killer Soda Pop

Greetings, fellow nerds.

If you consume soft drinks in the United States, chances are that you've noticed that this warning has begun to appear on two liter soda bottles. When I first saw it, I could hardly believe my eyes. I find it hard to imagine what possible circumstances could conspire to cause the cap to blow off with enough force to do bodily harm. So why the warning label? Does it really help, or is it just the brainchild of a misdirected torte lawyer?

I'm going to argue that the real dangers of soda come from drinking it, not opening the lid. I'm going to expose how skewed our senses of risk and responsibility are by showing that the high fructose corn syrup in soda does far more damage than its allegedly explosive top. This post will cover the risks of getting blinded by a rocketing soda pop top, while the next one will look into the health consequences of drinking that same soda.

The Letter Never Sent

When I set out to uncover the statistics behind the warning labels on soft drink bottles, I had imagined having to write to a representative of some bottling authority with a request for statistics on eye injury resulting from opening plastic bottles. I'd say my friends and I are casually interested in why these warning labels were deemed necessary, and ask for the reasoning behind the warnings.

I was kind of looking forward to composing the letter. Setting the right tone would be key: I'd have to come across as earnest while acknowledging the whimsical nature of my request. Usually, public relations people at corporations give you the benefit of the doubt and respond in good faith. However, it turns out injury statistics from soda bottles are well-known and published, so writing is unnecessary. I might still write that letter; if you'd like me to carry through with this plan let me know in a comment.

The Answer's Before Our Eyes

I'm not planning on writing the above letter because the British Journal of Optometry has already answered my question. In this publication (.pdf warning), titled “Serious eye injuries caused by bottles containing carbonated drinks”, they say “plastic and metal cans pose little danger: we found no related injury among the 12 889 cases.” Their database held combined data from the US, Hungarian and Mexican eye injuries from 1982 – 2002 (although it didn't have data for all countries and all years).

The paper's authors (F Kuhn, V Mester, R Morris and J Dalma) found not one instance of a soda pop bottle causing an eye injury in any of the countries in any year. It would be a stretch to say that absolutely no eye injuries happened because of pop bottles during the 1982-1999 period in the States (2000-2002 US data not published in Kuhn et al.) since the database of eye injuries used (USEIR) collects information of only serious eye damage, and not all states of the US contribute data to USEIR yet. Still, the fact that Kuhn et al. found not one case of plastic bottles causing eye damage over a 20 year span is suggestive that they're pretty harmless. If you know of someone who has been injured by a plastic bottle top, please leave me a reply about it.

Incidentally, champagne is not so benign. Kuhn. Et al. mention 43 cases of severe eye injury from sparkling wine corks to the face; the majority of these (37) were in Hungary. The remaining 6 wine cork injuries happened to Americans. If we assume the USEIR reflects only about half of all eye injuries, that's still less than 1 serious American injury per year. (Aside: Does anybody know what Hungarians put in their hooch that makes it so explosive? Kuhn et al. are baffled too.)

I'm Armed and Carbonated

Because I'm a physical sciences nerd, I'm going to figure out how dangerous pop bottle caps could be under ideal firing conditions. Typical soft drinks are pressurized to about 300 kPa. If the surface area of the cap is 3 square cm, the cap is under about 90 Newtons of force, or about 20 pounds. If we assume that some manufacturing error resulted in the cap suddenly becoming loose after a vigorous shaking of the soda at room temperature, this 90 Newtons would act on the cap for as long as the cap blocks the exit path of the gas: about 2 cm. If that 90 Newtons accelerates the cap for 2 cm, the energy transferred to the cap would be only 1.8 Joules. Most accidental cap accidents would be under less-than-ideal launch conditions, so I would be surprised if caps blew off with more than 1 Joule of energy.

How much is 1.8 Joules? About the same amount of energy as a snapped rubber band (one with a spring constant of about 250 N/m), if you stretch it three or four inches. That might sting if it hit you in the eye, but it's not likely to do permanent damage even with a direct hit. It probably wouldn't be too hard to get pop bottles approved as projectile toys for kids old enough not to try to swallow the cap.

In summary, my analysis suggests there isn't enough punch in a plastic bottle cap to do serious harm, and the lack of any evidence of eye injuries backs up my case. From here on in, I'm going to assume that the risk of serious injury from pop bottle tops to consumers (forewarned or no) is 0. Of course it isn't exactly 0, but the Kuhn et al. study covered about 5 billion person-years of exposure to pop bottle tops and didn't find a single serious injury, suggesting an upper bound on the injury rate of about 1 per 10 billion: close enough to 0 not to matter.

Time lost to the warning label

Even though an in-depth study failed to find even one instance of a plastic bottle cap damaging anybody's eye, one might think that it's better to abide by the precautionary principal, that is to warn people of the pressure danger in opening their soda on the off chance that we might prevent even one eye injury. I can imagine the person making the decision to put warnings on all plastic bottles would have a warm, fuzzy feeling if they saved the vision of even one child.

I disagree with the precautionary principal for two reasons. First, the cost of having warning labels is not zero: the precious seconds of our lives we waste worrying about inflated risks add up. Second, ubiquitous, petty, self-important warning labels can inure the public to warnings on genuine risks.

Time Lost Reading Gratuitous Fear-Mongering Messages

How many lifetimes have been wasted reading this warning message? Let's assume that, on average, every American reads this warning once per year. (I don't know if the rest of the world has adopted the warning label yet. Let's hope the insanity has been confined to the Home of the Mega Lawsuit.) If it takes about 10 precious seconds of your life to read through the text, that means that nationwide, about 95 years of life are wasted every year we insist of putting these gratuitous warnings on soda bottles. Equivalently, as you read this, you can expect 95 Americans are busy reading through the above text about how their life is potentially threatened by the intense pressure of the liquid refreshment in their hands.

Think of the good which could be done with those 95 person-years. Movies, novels and poetry written, walks in the park, vacations; all of these and more might have been accomplished if it were not for these unnecessary, attention-whoring portents of soda pop doom. There are some who balk at doing moral algebra. I think that 95 years of vision wasted by eye injury is in principal equivalent to 95 years of vision wasted by reading a warning label.

Since the net effect of the warnings is to decrease the visual lifetime of the consumer, I propose that the soda bottle companies stop printing the warnings. I also propose that we allow companies complete immunity from lawsuits arising from activities whose associated risk is so small that to post warning labels does more net harm than good.

Warnings, Warnings Everywhere...

... nor anyone to think. Perhaps the most damaging consequence of posting warnings on soda pop bottles is that we end up reflexively ignoring them. I hesitate to read every warning label I see (in truth, I didn't even notice this one below until this minute when I started looking for warning labels).

What's on the bottom of my keyboard? Is there a risk of electric shock? Perhaps deadly chemicals leak up from between the keys every midnight. How would I know? I plugged in my keyboard, it worked, and I'm happy. There's no way I'm going to pursue every last nagging, ass-covering warning I see out there, and that means I'll ignore warning signs when there are real dangers too.

I think we should standardize a color-coded warning label system, where different risk orders of magnitude have different colors. You could ignore anything less than a yellow then, unless you were exposed to the risk all the time. Manufacturers would be immune to lawsuits over correctly-labeled risks, and would be made liable for time lost if they grossly overstate the risk. I've started a list of color-coded risks - check out my wiki and add to it if you feel so inclined.

Conclusions

Gratuitous warning labels (such as the one on pop bottles) make us worry about the wrong things. There are genuine risks out there, but the system we have for making people aware of them is broken. We need to push through laws that protect companies from one-off lawsuits caused by the failure to alert customers of excessively-small risks, and then maybe we can talk about demanding that risk labels be removed when the opportunity costs or reading them outweigh the benefit they do.

EDIT: I'm adding information of the life expectancy decrease (LED) and equivalent driving distance (EDD) for opening a bottle of soda. See this post for an introduction to the LED and EDD.

If Americans open roughly 100 soda bottles per year, and if the annual risk of getting an eye injury from opening a soda bottle is less than 1 in 10 billion, the risk of opening each bottle is less than 1 in a trillion. That means that even if we were to assume any serious injury from a soda bottle were fatal, you life expectancy would decrease from opening a soda bottle by only 2.5 milliseconds (80 years - a typical lifespan / 1 trillion). Opening a soda bottle therefore has an LED of 2.5 milliseconds or less.

The EDD is computed by the fact that 14.6 driving fatalities occur every billion miles. The distance you'd have to drive to incur the same risk as opening a soda bottle is therefore 1 billion miles / 14.6 / 1 trillion; the EDD is less than 4.5 inches; so driving more than the length of a pickle is more risky than opening a soda bottle, once again with the ridiculous assumption that all bottle top eye injuries would be fatal.