What's that smell? (Indoor Air Quality).

When ozone comes in contact with carpets, unpleasant odors arise that differ from "new carpet smell." The malodorous fumes arise from the oxidation of vegetable-based machining oils found in carpets into compounds called aldehydes and ketones. Although the odors do not cause acute health effects, "they may influence well-being, mood, and productivity," says environmental engineer Glenn Morrison of the University of Missouri at Rolla.

While at the University of California at Berkeley, Morrison and civil engineering professor William Nazaroff exposed samples of commonly sold carpets to 100 parts per billion (ppb) ozone for 10 days and continuously measured gas emissions. As reported in the 15 May 2002 issue of Environmental Science & Technology, a variety of aldehydes and ketones were detected. Particularly noted was a form of the aldehyde 2-nonenal that the human nose detects, at just 100 parts per trillion (compared to 500 ppb, or 5,000-fold higher concentrations, for pine and lemon scents). Morrison describes the smell of 2-nonenal as "pungent cucumbers." Other aldehydes and ketones smell like cut grass or nail polish remover. "The whole suite of odors could make a person ask `what's that smell?' when they walk into a room," he says.

Indoor ozone concentrations rarely reach 100 ppb--a Los Angeles home with open windows on a summer afternoon contains about 40 ppb--but the researchers used high concentrations to speed reactions and generate detectable levels of smelly chemicals. At lower ozone concentrations, reactions proceed more slowly.

About 80% of reactions occur in the carpet's fibers, and the rest occur in the backing. Carpets become a reservoir for aldehydes and ketones, and odors may linger for years. In contrast, "new carpet smell" stems from carpet glues, and odors dramatically drop a few weeks after installation.

The strong, irritating smell of ozone pales beside the unpleasant odors that result when ozone reacts with common everyday substances. According to a review in the December 2000 issue of Indoor Air by Charles Weschler, a chemist at the University of Medicine and Dentistry of New Jersey/Robert Wood Johnson Medical School in Piscataway, many common indoor substances, including pine- and lemon-scented cleaners, react with ozone to generate indoor pollutants such as formaldehyde. Weschler says ozone also reacts with indoor surface materials such as rubber, neoprene, and paint, releasing compounds that can damage both building materials--for example by causing rubber to crack--and human health.

Ozone-linked indoor pollutants may contribute to "sick building syndrome"; studies have found that chemicals present at odorous but nontoxic concentrations cause headaches, mucous membrane irritation, dizziness, and irritability, and reduce productivity and raise stress. "The evidence for adverse health effects from [ozone-related] carpet emissions is indicated, yet still circumstantial," says Morrison. Weschler adds that the human health effects of odors from carpets "need a great deal more attention."

Opening windows to air out smelly carpets may actually make the problem worse by adding more ozone. To limit indoor ozone, Weschler recommends opening windows at night when outdoor ozone levels are lowest. Air conditioners can help--they largely recirculate inside air, rather than drawing in outside air. However, limiting ventilation concentrates pollutants that originate indoors.