The study, scheduled to appear online in the American Chemical Society's Environmental Science & Technology (ES&T) journal, classified 17 out of 62 printers as "high particle emitters" because they released so much toner powder into the air. One of the printers released ultra-fine toner particles at a rate comparable with cigarette smoking, according to the American Chemical Society.

Not all printers deserve warning labels, however. Thirty-seven of the 62 printers tested did not release enough particles to reduce air quality. Six released low levels of particles and two released medium levels.

The study included Canon, Hewlett-Packard, Ricoh, and Toshiba printers sold in Australia and the United States. It was conducted by Lidia Morawska, a professor at Queensland University of Technology in Brisbane, Australia, and colleagues, with funding from Queensland Department of Public Works and The Cooperative Research Centre for Construction Innovation.

"The highest printer particle number emission rate found in the chamber study was 1.6 * 1011 particle min-1, which is close to the median value of submicrometer particle number emission rates for activities, such as cigarette smoking (1.91 * 1011 particle min-1), occurring in residential houses," the study said. The high emitter list includes one Toshiba model and the rest of the listed printers are manufactured by HP.

The study notes that printer emissions are highly variable and that some printers, such as the HP LaserJet 5, can be either non-emitters or high emitters of particles in different circumstances. "The high standard deviation of the average emission rates estimated in this study also indicates that the particle emission process and the behavior of individual printers are complex and that they are still far from being completely understood," the study said. "Many factors, such as printer model, printer age, cartridge model, and cartridge age may affect the particle emission process and all of these factors require further study."

Morawska said that her group discovered printer pollution by chance. "It wasn't an area that we consciously decided to study," Morawska told the American Chemical Society news service. "We came across it by chance. Initially we were studying the efficiency of ventilation systems to protect office settings from outdoor air pollutants. We soon realized that we were seeing air pollution originating indoors, from laser printers."

Morawska and her associates found that indoor particle levels increased by a factor of five during work hours, when printers are used. New toner cartridges and graphics-intensive print jobs added to the amount of toner particles in the air.

The inhalation of ultra-fine particles can affect human health in different ways, depending on the material inhaled and the quantity. But such particles can cause respiratory irritation or more serve conditions including cardiovascular problems or cancer.

According to the Environmental Protection Agency, "Small particles less than 10 micrometers in diameter pose the greatest problems, because they can get deep into your lungs, and some may even get into your bloodstream."

Current toner particles typically range from 8 to 10 micrometers, according to Wikipedia.

The toner particles measured in the study ranged from 15 to 710 nanometers, or 0.015 to 0.71 micrometers.

HP's Material Safety Data Sheet for its Color LaserJet 8500 Series suggests the risk from toner inhalation (for that particular model at least) is not particularly significant. "Minimal respiratory tract irritation may occur with exposure to large amounts of toner dust," the document states.