Research at Harvard University has shown that, during printing, metal-oxide nanoparticles (defined as 100 nanometres and smaller, hence 0.1 micrometres and smaller) are released into the air from toner-based laser printers and multifunction devices. These machines use toner particles that are on average 20 micrometres in overall diameter, but the surfaces of the toner particles themselves carry innumerable tiny metal-oxide nanoparticles. These ultra-small metal-oxide nanoparticles are highly bioactive and could cause harm to the lungs and also elsewhere in the body, given that particles 0.1 micrometres and smaller can cross biological membranes (including lung alveoli), thereby gaining access to all organs via blood circulation. This remains an area of active research, with many knowledge gaps.