From the Correlation does not Equal Causation Files

On this blog, I have often felt the need to point out that correlation does not equal causation.  For example, if X increases at the same time Y increases, it is not necessarily true that X causes Y or Y causes X.  The correlation could be a coincidence, or it could be that both X and Y are related to a third variable Z that drives their movement.

Anyway, I see this mistake all the time.  What I did NOT expect to see was that someone would have to explain that non-correlation does not equal causation.  But that seems to be the wacky world that environmental science has descended into, via the Commons Blog:

EPA's new report "America's Children and the Environment" notes that
air pollution declined, but asthma prevalence continues to rise. One
possible conclusion from this is that air pollution is not actually a
cause of asthma. In fact, that's the most plausible conclusion. Every
pollutant we measure has been dropping for decades pretty much
everywhere, while asthma prevalence has been rising pretty much
everywhere. This is true throughout the entire western world, not just
the U.S. In fact, asthma incidence is highest in countries with the
lowest levels of air pollution. Asthma is rare in developing countries
with much more polluted air. Asthma incidence is simply unrelated to
air pollution. Asthma attacks are probably unrelated as well. But even
if air pollution can cause asthma attacks, it is a minor cause,
responsible for less than 1% of all asthma attacks.

Despite these two trends going in the opposite direction, environmental activists still insist that large increases in asthma rates are driven by pollution:

A report by E&E News
(subscription required) makes it clear that what's in EPA health
reports doesn't actually matter. The story opens with "While the number
of children living in areas violating ozone and particulate matter (PM)
standards has declined in recent years, adolescent asthma that results
from exposure to such pollutants continues to rise, according to new
U.S. EPA statistics." The journalistic goal is to raise health alarms,
whether warranted or not. Thus, the news story itself says air
pollution, the presumptive cause of asthma, went down and yet asthma
prevalence went up. However, the reporter claims air pollution is
responsible for rising asthma just the same.

Wow.  These guys could be the poster-children for refusing to adjust their beliefs in the face of actual facts.  They even acknowledge that pollution and asthma are going in opposite directions and still they insist on their causation theory.

ostscript:  I am willing to believe, maybe, that there is some unknown, unmeasured and unregulated pollutant out there that is increasing and is causing increases in asthma.  However, that is not the argument these folks are making - they are using asthma increases to lobby for tougher standards on known pollutants.

Update:  The best guess I have for the increase in asthma in this country, and the strong positive correlation between asthma and economic development, is that it has something to do with indoor pollution.  The spike in asthma cases seems to parallel the rise in energy prices.  Beginning in the 1970's, we began sealing up houses tighter and tighter to conserve energy.  Increasing penetration of air conditioning simultaneously caused people to close the windows.  I am convinced its something inside, not outside.


  1. Dan:

    I'd be willing to believe that asthma and pollution is even inversely related to an extent. The theory I saw in newspapers in Japan about a year ago (which has recently had huge increases in asthma) was that all of the air purifiers everyone's got are weakening the natural defenses kids would normally build up while they're developing.

  2. Noumenon:

    I feel like that postscript was directed straight at me, because that was just the question I had -- so you did a good job of convincing!

  3. markm:

    Allergies are your immune systems' response to not having enough real germs to fight.

  4. Rob:

    Mr. Coyote, isn't it obvious why asthma is increasing:
    1. Increased divorce rate in the USA
    2. Increasing stock market
    3. Increasing casualties in Iraq
    4. Decreasing cost of OREO cookies and/or healthcare
    5. Increasing amount of bureaucracy required to run a business
    And the main cause:
    6. Increased number of 'illegal aliens' in the USA

    That's 6 facts which are causing the increase of asthma !!! :P ... lol

  5. Stuart:

    Is it possible that air pollution in the near past reached some critical level and that as more kids are growing up with this critical level, when they are younger and more vulnerable, they are developing asthma, leading to increasing numbers of asthma sufferers.

    I am not a scientist but I guess you would have to go back and see the rate of air pollution over time. People who were born before X year, when pollution was lower, might not have been affected when they were young and not have developed asthma.

  6. Bill:

    I don't think Stuart's suggestion is valid...pollution levels have been dropping for most of my lifetime (I can remember when many households were heated by coal, and the smell of coal smoke was a constant in the winter).

    How about the possibility that we do too good a job of protecting children from normal insults, and that their immune systems become over-reactive and unable to provide a normal response to environmental challenges?

    What is very clear is that the environmental community is stuck on a mind set that has led to an automatic rejection of data that fails to fit the politically correct meme. And that is costing us much in wasted effort and even more in failing to deal with reality in a timely fashion.

  7. Teri Pittman:

    I've read that one of the reasons for the rise in asthma is that cities plant only male trees. (And yes, there are male and female trees.) The male trees continue to put out pollen which really has no place to go but in the air. The female trees aren't planted as they are too "messy".

  8. dearieme:

    I wonder about indoor air pollution. When I was growing up, asthma was rare in Britain but the great smogs were just ending. (My wife saw the last one in London, a cousin the last in Edinburgh.) Outdoors air is much cleaner now. But over the years our houses have become almost sealed - no more cool, draughty homes for us. But that may mean that they are under-ventilated. And, of course, children spend far less time playing outdoors.

  9. Jim Collins:

    Has the number of Asthma cases really increased or has the diagnosis changed? I seem to remember that the level of over weight children seemed to be acceptable until the definition of what an over weight child changed. Are more people developing Asthma or are doctors getting better at diagnosing it?

  10. DaveJ:

    Given the over-regulatory stance of the entire government structure, the cries of alarm at even ordinary risks, and the vested interests on the part of scientists, regulators and nannies, I am not ready to accept the existance of an 'unknown, unmeasured and unregulated pollutant'.

    We have an army of motivated people discovering, metering to parts-per-billion, and over-regulating everything. It would be safer to assume that they have already determined all the real risks, the way they're inventing trivial ones.

    . . .

    OK, they might have missed a couple, but I don't think thats the way to bet.

  11. markm:

    When you focus on indoor pollution, there's no shortage of suspect allergens: formaldehyde and other gases seep out of plywood and other building materials, dust mites, cockroach dander (not common in middle class homes, but it's the slums where asthma is most common), cooking fumes, mold, etc. So kids staying indoors more and houses being sealed tighter could make asthma more common, if it's rate of occurrence actually correlates to the exposure to allergens.

    However, if exposure to allergens by that hypothesis, our unwashed ancestors should have all died of severe asthma. They lived in crude huts, moldy castles, and even caves, heated by wood fireplaces with poorly designed chimneys (and sometimes no chimneys at all), and inhaled more smoke than a non-smoking city resident. They didn't just breathe a few insect parts, insects crawled upon them, biting and laying eggs under their skin. Pollen blew right through their dwellings.

    So, it makes at least as much sense to blame allergies on chronic underexposure to allergens, leaving the immune system unable to learn to limit its response to be proportional to the threat. But I think stress and limited exercise are also major factors.

    When our ancestors stressed-out, there was generally a physical reason for it, and often the proper response involved physical exertion: running away from the hungry bear, fighting barbarian raiders, catching and killing the rabbits that are tearing up your garden, working frantically from can-see to can't-see to get the crops harvested. Now, stress is usually caused by subtler problems, and people often have to sit around and stew, with adrenaline running through the blood, but no call for the physical effort it prepares them for. So the body's defenses go on the alert, and there's nothing real for them to fight - but, like a government agency whose reason for being has gone away, they'll find something...