A Breath of Fresh Air
As I write this, I'm about to embark on a week-long walk through the alpine country of Yosemite. The most exhilarating early moments of these trips come when I take a deep breath of the pine-scented air, which tells me I've arrived. But as with most other people, most of my time is spent indoors, at home, in the office, in the car. We spend as much as 80 to 90% of our modern lives within the confines of the built environment. Perhaps it's the contrast which makes the mountain air smell so clean.
We know we need to bring fresh air into buildings. But fresh air comes at an energy cost because that air usually needs conditioning. In the west the cost is mostly from heating or cooling, but people east of the Rockies often must deal with humidity, which is a great deal more energy intensive. In the early 1980s attempts to save energy led to reductions in outdoor air introduced per person, down to 5 ft3/min (CFM) as a standard, and in practice often much lower. About this time complaints of sick building syndrome began to emerge. Eventually ventilation standards rose to about 15 CFM per person. The complaints, while still heard, were less frequent.
Where did that CFM number come from? Was it based on some experimental results, or epidemiological studies? Well no, the standard, developed by the ASHRAE 62 committee, is a consensus standard, meaning that, while there's no hard data to support it, as compared with say, 16 CFM, the committee of experts agrees that it-seems to work. As Steve Taylor, who has been involved with the standard over the years, has said, “They pulled it out of the air.”
Could we do better? That would be difficult, because there are so many variables confounding a health study. One thing we could do is identify criteria pollutants, which are specific compounds associated with health effects, for which exposure limits have been developed. Thousands of these exist, but several are commonly found at high levels inside buildings.
Formaldehyde is a prime example. Used as a component of glue and adhesives, it's found in everything from particle board to fiberglass insulation. Unfortunately it doesn't stay in these products, but persistently out-gasses over a long period of time. OSHA, the Occupational Safety and Health Administration, sets a limit of 0.15 parts per million (PPM) exposure limit over an eight-hour period, with a maximum permissible limit of 2 PPM in 15 minutes.
Trailers supplied by FEMA to victims of Hurricane Katrina famously resulted in many reports of throat and lung irritation, and were found to have average levels of formaldehyde at 0.075 PPM two years after their initial deployment. While formaldehyde itself has health implications, it reacts with ingredients such as ozone to form other reactive compounds. And while building materials and furniture are important sources of formaldehyde, other common processes taking place inside buildings, such as cooking, also produce formaldehyde. So what are we to do?
The first and most obvious step is to limit the sources. Insulation can be purchased which doesn't contain formaldehyde. Natural wood emits decidedly little formaldehyde, and plywood out-gasses much less than oriented-strand board or particle board. Phenolic glues used in exterior-grade plywood emit minimal formaldehyde, and cost only a little more, so substitution of glues is an option.
Beyond source control, ventilation is the obvious way to mitigate such problems. Loose building envelopes obviously provide a source of fresh air, but this is not necessarily the form we should want. The rate of ventilation through cracks and holes is largely a function of wind pressure. Often, periods of high winds coincide with rain, snow and cold weather, occasions when we're not looking for extra ventilation. Likewise, a lack of wind doesn't mean we want to bask in indoor air pollutants.
Particularly as building envelopes are tightened to save energy, mechanical ventilation becomes a more and more important means of ensuring good air quality. This sort of air replacement need not consume much energy; a continuous bathroom fan can consume only 10 or 15 W and provide a reliable amount of fresh air. California Title 24 now requires continuous mechanical ventilation for new homes.
Another important strategy is to make sure contaminants are effectively removed. Kitchen range hoods don't necessarily effectively capture cooking effluents. It's important to get enough airflow, but high volumes are less important than a hood which extends over the cooking surface adequately to catch the rising fumes. Equally important is the protection of the rising airstream from side currents which blow it out from under the hood. This makes island range hoods challenging, and down-flow capture even more challenging.
All in all, we might not be able to make our homes as free of pollutants as those alpine meadows, but with careful control of indoor sources and a bit of effort to contain and remove the remaining pollutants, the smoke won't get in our eyes.