This somewhat worrying study, Laser printers as bad as cigarette smoke, explains that some laster printers are outputting a large number of particulates in the form of unused toner.
Those particles are precisely what you don't want if you have any sort of breathing condition, like asthma, or even, to be honest, if you don't.
According to the article, a large number of HP printers are in the list that output a high number of particulates. Laser printers were