60% of all statistics are crap

In last Thursday's Home and Garden section of the LA Times I found the following quote in an article entitled Breathing a Little Freer Indoors.

"Over 60% of the air you breathe in any closed space is off-gassing from surface materials," says Ellen Strickland, owner of Livingreen stores in Culver City and Santa Barbara that sell environmentally friendly home products. "It's an accumulative effect of everything that's on the walls, furniture, counter surfaces, your clothes, the curtains -- anything that's brought into that space."

While I agree that VOCs are undesirable, I find it impossible to believe the 60% figure quoted above. I think that the greenies do themselves a disservice when they bandy about patently false statistics.

Generally, scientists get concerned over known carcinogens at the ppb or low ppm level (parts per billion or parts per million). One ppb is 0.000000001 or 0.0000001%. One ppm is 0.000001 or 0.0001%. The 60% level quoted in the story is 600,000 ppm. You wouldn't be able to breath in air from which 60% of the oxygen had been displaced!

Let's do a little Google searching and some algebra to figure out a reasonable number for a bound on the potential percentage contamination of VOCs.

A high level of VOC contamination assumed in an EPA study that I found online was 0.27 mg/m^3. Let's assume this is typical. Air has a mass of 1.2 kg/m^3 at room temperature (surprisingly high, isn't it?). The mass ratio of 0.27e-3 / 1.2e3 is going to be in the fraction of a ppm. Allowing for the typical larger mass of an organic molecule from VOC (perhaps 100 AMU) compared to air (N2 = 28 AMU) we can scale the typical mass ratio to a molecular ratio and we get an even lower number, perhaps 50 ppb. 50 ppb is 0.000005%, if I've managed to move the decimal place appropriately. Scale this number up or down, depending upon your going-in assumptions. No matter what you do, you won't get even close to 1%, let alone 60%.

Given the innumeracy in society at large, is it surprising that number like 0.000005% gets turned into 60%? It ought to be. By the way, it took me not too long to dig up these numbers - the results were all on the first search page. Newspaper editorial staff would be advised to do likewise.

So, should you be concerned about 50 ppb volatile contamination? Absolutely! Particularly if it's chronic exposure to the wrong stuff. Should you believe 60% contamination figures tossed about by the LA Times or by proprietors of green living stores? Absolutely not.

The LA Times later redeems itself with the following:

Santa Cruz architect Hal Levin has spent nearly 30 years researching building ecology, a term he coined in 1979. He was interested in an environmentally friendly cork veneer widely used by green designers, so he had it tested.

The material was supposed to meet the European standard of 0.1 part per million of formaldehyde, which already was six times higher than standards for California state office buildings, he says. Test results showed that the emissions were five times higher than the European standard, or about 30 times California's.

But they never circle back to the 60% figure or compare it to 0.1 ppm.

As a sanity check, here's some Australian guidelines for common gaseous contamination (also found after a brief web search). Even the sum of all those PPMs would not get close to 60%.
carbon monoxide 9.0 ppm (parts per million) measured over an eight hour period
nitrogen dioxide 0.12 ppm averaged over a one hour period
0.03 ppm averaged over a one year period
ozone 0.10 ppm of ozone measured over a one hour period
0.08 ppm of ozone measured over a four hour period
sulfur dioxide 0.20 ppm averaged over a one hour period
0.08 ppm averaged over a 24 hour period
0.02 ppm averaged over a one year period

1 comment:

  1. I scratched my head when I read that statistic, too. I don't know about you, but ~80% of the air I breathe is molecular nitrogen. The rest is almost all molecular oxygen with a few trace gases.