Toner is a powder used in laser printers and photocopiers which forms the text and images on the printed paper. In its early form it was simply carbon powder. In order to improve the quality of the printout the individual carbon particles were blended in a polymer. The polymer particles can be melted by the heat of the fuser, causing it to bind to the fibers in the paper. The exact polymer varies by manufacturer but might be a Styrene Acrylate Copolymer or a Polyester Resin.

The original particle size in toner averaged about 12 micrometres. When image resolution was improved to 600 dots per inch, however, the particle dimension was reduced to 8 micrometres. Further reductions in size will be necessary for improvements in resolution. Toner manufacturers maintain a quality control standard of these particle sizes in order to produce a powder suitable for use in printer devices; the particle size is not allowed to vary too much from the required dimension.

In earlier machines toner was shaken from a bottle into a receptacle in the machine. Modern methods use a disposable cartridge. These discarded cartridges are often refilled by third party vendors.

As a fine powder, toner can remain suspended in the air for some period, and is considered to have health effects comparable to dust. It can be an irritant to people with respiratory conditions, such as asthma or bronchitis. Following health studies on bacteria, the carbon content of the toner has been reduced to the point where it is not considered a cause for concern. Some toner is also known to be a carcinogen, but such a large exposure is required that it is generally considered harmless. If you get toner on your hands, be sure to wash it off with cold water. Hot water will open the pores on your hand and the toner will stain your hands.