Friday, May 29, 2009

Relative safety of stairs and swimming pools

[Post updated: A friend called BS on my previous estimate of the number of houses falling into the CPSC's "Stairs, Ramps, Landings, Floors" category, so I've fixed that. The change affects the magnitude but not the polarity of the bottom line.]

As we're finishing up our swimming pool, my wife and mother-in-law and I were naturally led in a recent dinner conversation to consider whether the pool is dangerous. This is, of course, an ill-posed question.

So I changed the discussion to which of the stairs or pool was more dangerous. The stairs typify a common threat which which everyone is familiar. The pool typifies a threat for which there is plenty of hype.

I was able to remember the gist but not the exact numbers in my previous blog post on this subject. Now see, there's the value of my blog (I knew it was going to pay off someday!) -- I have a nicely written set of notes available online to which to refer. Sadly, I had not completely anticipated my mother-in-law's argument, so here's an update:

There are 8.6 million swimming pools in the United States, and 116 million homes. If we make an approximation that all those swimming pools are residential, 5322 deaths/year for 8.6 million residential pools is 62 deaths/year per 100,000 houses with swimming pools.

I'll assume that essentially all houses have stairs, ramps, landings, or [more than one] floor. That means the 202,104 deaths/year for 116 million homes equates to 174 deaths/year per 100,000 houses.

Since we have both a pool and staircases, my best estimate is that our stairs are 2.8 times more likely to kill someone than our pool.

The difference may be substantially larger. Our pool will have modern safety features like an automatic safety cover, parallel separated drains, and a properly engineered diving envelope for the diving board, along with a raised-periphery design that makes snapping your neck on the bottom at least very awkward.

Our stairs, on the other hand, are very much like stairs everywhere, and thus should be about as risky. I suspect that a disproportionate number of "Stairs, Ramps, Landings, Floors" fatalities are concentrated in the portion of houses with actual staircases, and so my estimate above understates our staircase risk. The two most used of our three staircases have turns part-way down, which I think makes them marginally safer since you are less likely to fall all the way down the flight, but I doubt that affects the polarity of my argument.

Bottom line: no, I'm not worried about the safety of my kids around the pool, but I have gotten noticeably more nervous about the stairs since running these numbers.


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  2. Hi,

    I came across your blog because I am building a pool myself. Thanks for the info. Regarding your stairs argument one thing to consider is the majority of deaths because of stairs are the elderly. They break easy. So a larger percentage of elderly are using stairs than swimming. In general most people swimming are younger. So one could argue stairs are extremely dangerous for elderly and pools are dangerous for the very young.

    My take away.. if you are old buy a one level house.. and if you have young kids make sure they can swim before you get a pool.



  3. Hey Max,

    The CPSC stats ( agree with you: it's moslty the 5-14 year old crowd that get injured (and presumably killed) in pools.

    The CPSC stats also show that stairs injure more older people than others, but these injuries are hardly the majority. I don't have information on death distribution rather than injuries. Injuries are spread out among the whole population quite a bit. The safest group, those aged 5-14, are only three times less likely to be injured than the 65+ group.

    As I've said before, the really big tilt is between men and women: 889/100,000 women are injured every year on stairs, versus 556/100,000 men. That difference is incredibly large. There is a 71% chance that during our 20 year stay at this house, one of my daughters or wife will be reportably injured on our stairs, and a 7.2% chance that one of us will be hospitalized or killed by an accident on the stairs.

    It reminds me of how Douglas Adams described Vogon spaceships: deadly like a brick wall built across a motorway.