In July 2007, an Australian research team quietly released a study that alleged that laser printers could release a fine, toxic dust -- "on the scale of inhaled cigarette smoke" -- whenever they were in use. Certain model printers were noted as being far better than others, but the upshot was that many office workers were probably being exposed to some nasty airborne particles that they really had no control over. The story exploded (and it became one of the biggest stories on Yahoo! Tech that year). Printer companies, of course, were not entirely thrilled with this information and disputed the findings vigorously.

Nearly two years later, the same researchers are back... with a vengeance. Their goal: To determine why some printers produced so many particles and some produced considerably fewer. Rather than test a few dozen printers, this time the research focused on two printers (both from HP): One known to produce a very small amount of particulate matter, and one which had been shown to produce about 1000 times as much. That's an enormous range for two printers from the same manufacturer.

To get to the point, here's what the study found: The temperature that the toner, lubricating oil, and certain internal printer components reach (and how fast they reach it) is what is largely responsible for causing the rise of particulate emissions. Once certain thresholds were crossed (which vary depending on the component), particulate emissions went through the roof. The study also found that toner itself isn't the real issue, but rather a complex secondary reaction that occurs in the air and involves organic compounds that originate on paper as well as toner, also involving airborne ozone, to produce the resulting emissions.

Without experimenting, there's no easy way to guess when these temperature thresholds are crossed, as temperature varies up and down during a print job, and not always in ways you'd expect. In the tests, both printers' temperature (and emissions) peaked in the first minute of a print job and then declined, but for the "bad" printer in the test, a second peak occured after two sustained minutes of printing. Ultimately, the study found that how well the printer controlled temperature levels was what was responsible for its emissions level. The report concludes by suggesting that printer companies should focus on this temperature issue in order to make their products potentially less dangerous to those who work around them.

Ars Technica has much more in-depth coverage of the study. The complete text of the report is available online as well, at a price of $30.

(And to attempt to stem the flood of email that this blog post is certain to generate: Remember, inkjet printers do not suffer from this problem and are generally considered "safe." And if you do have a laser printer, I unfortunately can't tell you how safe or dangerous a specific model might be.)