Will Higher Ventilation Rates Reduce Student Absences in School?
A new study from Lawrence Berkeley National Laboratory (LBNL) makes a bold claim that increasing ventilation rates can cut the number of student absences related to illness. The lead author on the paper, Mark Mendell, is quoted by LBNL's News Center as saying, "Our overall findings suggest that, if you increased ventilation rates of classrooms up to the state standard, or even above it, you would get net benefits to schools, to families, to everybody, at very low cost." But what did they have to assume to find that suggestion?
Looking at ventilation rates and illness absence
Their study was published in the refereed scientific journal, Indoor Air, and is titled, Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools. What they did was pretty simple, really. They put dataloggers with carbon dioxide sensors in 162 classrooms of 3rd, 4th, and 5th graders. The classrooms were in 28 different schools in different parts of California. The dataloggers measured CO2 levels and turned them into averages for each 5 minute period. The researchers monitored the CO2 data for two years.
The next step was turning the CO2 levels into ventilation rates. Since people breathing indoors raise the CO2 level, the researchers developed a mathematical model to convert the amount of CO2 in the air into a ventilation rate. The higher the CO2 level, the lower the ventilation rate, and vice versa. (From the News Center story, I couldn't tell exactly how they did this, and I don't have a copy of the actual paper.)
When the researchers looked at their calculated numbers, they found that ventilation rates in California schools are mostly below the code-mandated rate of 15 cubic feet per minute (cfm) per person. The table below shows what they found. It's in liters per second rather than cfm. The reference point for your conversions is 15 cfm = 7.1 l/s, so you can see that only one of the lines below shows a rate above the required level.
After converting CO2 levels to ventilation rates, the team compared these rates to the classroom-by-classroom illness absence data. What they found was that as the calculated ventilation rates went up, illness absences went down. Combining the data from all the schools, they found that illness absences decreased by 1.6% for each additional 1 l/s of calculated ventilation. According to the Center News article, the illness absences keep decreasing all the way up to a ventilation rate of 15 l/s (30 cfm) per person.
That's an interesting result! Where they went with that result, however, is a bit surprising.
Extrapolating from correlation to causality
One of the biggest traps in science is to confuse or conflate a correlation, two things that change together, with causality, a relationship in which one change causes another. It makes up a significant portion of what is called pseudoscience, and it's also great for humor. Perhaps you've heard that global warming is caused by the decreasing number of pirates?
The abstract of the article states that the authors assumed causality from the correlation they discovered between calculated ventilation rates and illness absences: "Assuming associations were causal and generalizable..." The News Center article quotes Mendell on the topic: "We saw a correlation, but we don’t know if it’s directly causal." That didn't stop him from saying, as quoted at the beginning of this article, "...you would get net benefits to schools, to families, to everybody, at very low cost" by increasing ventilation rates.
OK, let me get this straight. They found a correlation between two variables and then went on to extrapolate all kinds of benefits based on their assumption that higher ventilation rates cause fewer illness absences? Yep. They sure did.
And this was in a refereed scientific journal? Yep again.
Too many questions!
As I said above, I haven't read the actual paper or seen all the data. Maybe I'm all wrong about this, but the case they made for causality in the News Center article and their research paper abstract was...well, they didn't make a case at all for causality. They just assumed it.
I think everyone in the field of building science understands that ventilation rates are related to indoor air quality (although they're not the only factor) and certainly can have an impact on health. My problem with what they've done here is that they should have concluded their paper by raising the many questions that still need to be answered:
How good is the model for calculating ventilation rates from CO2 levels? If they measured actual ventilation rates, how close would they be to their calculated numbers?
What other variables do we need to account for? Poor kids get sick more than kids from families with more money, but I didn't see anything about that in the article.
What does the 21 day lag between changing CO2 levels and illness absences mean?
There are just too many questions left open to be rushing to judgment about doubling the required ventilation rates in schools. A scientific paper in a refereed journal should not make recommendations based on assumptions. What can happen in such cases is that, because they appeared in a scientific journal, those recommendations get picked up by media and policymakers as having a validity they just don't have.
Maybe in a few years we'll have enough data to back up their claims, but for now, it's way too early to turn their data into policy.
Correlation does not imply causality.
Photo of school children by US Department of Education from flickr.com, used under a Creative Commons license.