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Laser printers and photocopiers are generally safe to use, or no more hazardous than a TV. However as with most devices there are some issues.
Laser. 
Laser printers commonly have a label inside saying "Caution- Class 1" or "Caution - Class 2 Laser".
The laser in most printers is not powerful enough to burn skin or pose a fire hazard but it certainly could damage someone's eyes.
The idea is generally that Class 1 lasers are sufficiently low in power to be safe. A laser can be Class 1 for two reasons:
- It is inherently low power - this is true for some printers
- Cut-out switches make accidental exposure to the laser normally impossible
Because the laser is guarded by cutouts it might be the least dangerous aspect of a laser-printers operation. CD and DVD rewriters pose similar problems.
Electrical. 
Laser printers use high voltages for toner transport - typically of the order 600 through to 6,000 volts. The electrical safety hazard is no greater than for a TV set. Laser printers are invariably mains driven because the fuser has to be made hot. The printer innards use high voltages. Standing a vase of flowers or a mug of tea on top of a printer would be foolish. If a printer does suffer a spill of water, don't touch it, turn it off at the wall immediately. (Providing the switch did not get wet). Let it dry out thoroughly and get someone with technical knowledge to look at it.
Allowing water into a machine whilst it is turned on will almost certainly do electrical damage.
Fire.
Some fast laser printers contain a Class 3 b laser which is not generally a fire hazard but is capable of igniting some materials. Interlock switches prevent the laser being operated with covers removed but if the switches are foxed for test purposes then a hazard is possible.
The toner itself is a fine plastic powder which can remain suspended in air for some time. In great concentration toner dust can be explosive. The only circumstances where this is at all likely is if a large toner spill is cleaned up using an ordinary vacuum cleaner.
Laser printers do contain a small heater in the fuser. In theory the fuser could get out of control and become a fire hazard. To prevent this manufacturers put a low melting point fuse and a thermal cutout on the fuser body - if things really do go wrong the fuser should shut down. Don't leave a printer turned on under a dust cover - parts like the power supply could overheat.
In the event of a fire involving a laser printer use a CO2 extinguisher.
Toner.
The unique thing a printer does is to handle ink. The chemical composition of the consumables and particularly of laser printer toner might be a concern.
Laser printers and photocopiers are common; so most people now have some exposure to toner. The toners used in older machines were potentially hazardous to health, but manufacturers became aware of the problem and either minimise exposure using cartridges, or claim that their own brand of toner is safe.
Toner dust.
Most toner dust is probably a mild health hazard. The material is a plastic and a colourant - black toner will be carbon black and styrene. Toner is sufficiently fine to wash into pores on the skin, and to remain air-borne for some time and be breathed in (its a source of VOCs). Because any powder has a huge surface area it will easily gives off any volatile component in its makeup. Some recent toner materials intended for machines with a 2400 dpi resolution contain components smaller than the "PM10" particles from diesel engines, considered to be a hazard to sufferers of asthma and emphysema.
Massive exposure to toner dust in a recycling workshop might potentially be a cause of mesothelioma so ventilation through a filter would be advisable.
Experience of accidental exposure to toner during engineering work suggests that it is possible to be made mildly ill by massive exposure to toner-dust, but such exposure only occurs when a large quantity is spilled and blown about. (The printers had been in offices hit by a bomb blast)
Lower level exposure through casual use of laser printers does not generally seem to be a problem but might become so (see the next paragraph). Photocopiers based on the Xerographic principle have been quite common since 1960 and printers since 1984 so there has been some time for any epidemiological evidence to accumulate.
Never use a standard vacuum cleaner for toner – it will penetrate the pores on a standard vacuum cleaner filter bag and be blown out in the exhaust. Worse still, a strong mix of toner in air can explode. Scoop up large quantities by hand, wearing protective gloves and using a paper bag or envelope that can be sealed before disposal. Disposal of compacted toner can probably be done in ordinary garbage if it is wrapped in a couple of plastic bags to prevent leakage. It is basically just plastic.
Cleaning up traces of toner can be difficult. The best method is probably to use foaming cleanser or soapy cold water on tissue paper - spray the cleanser onto the tissue, not onto the toner, because it is easily blown around.
Toner powder does not mix easily with water - fine powders often do not because of their large surface area. Toner is plastic, so it will tend to float. A damp soapy cloth does seem to pick it up toner quite well. Do not use hot water – the toner powder will melt and adhere to clothing and skin. Avoid solvents, most toner powders break down in solvents leaving a stain.
Fresh toner may be less of a problem than spent material. The toner collected in the waste bottle also contains a certain amount of material abraded from the drum - which might be an OPC material. OPCs contain metalic dopants, which may contain cadmium or selenium sulphide. Cadmium compounds in particular should be avoided.
Airborne toner.
A study by the International Laboratory for Air Quality and Health at Queensland University of Technology, Brisbane by Congrong He, Lidia Morawska and Len Taplin found that most printers do not emit any submicrometer particles but that 40% emitted some particles and 27% were high particle emitters. The high emitters tended to be recent models with microfine toners and the levels of emission close to the printer could be comparable with smoking as a health hazard.Since the problem only occurred with some printers manufacturers could presumably study and eliminate the problem. Otherwise the suggestion would be that printers shouldn't be located next to people and that the area round a printer should ventilate to the outside where dilution will reduce the problem.
Vapours.
The laser printer fuser melts toner into the paper as part of its operation. Melting a plastic generally gives off some vapour - presumably components like the plasticiser. So far as is known there is no indication that the vapours can be harmful but as with anything else uncertain it may be best to minimise exposure. Ventilation is the key to this. Don't use a laser printer in an unventilated room.
Developer.
The iron filings used in conventional developers are unlikely to be a hazard. We have no information on the resins used in some recent developer designs.
Ozone.
Ozone is no longer a major worry because most office laser printers produce little or none. Older printers and copiers did produce it in significant amounts.
Laser printers use an electostatic process to create the image on the drum. Several hundred volts of charge difference between the photo-conductor drum and the developer-toner mix are needed to stick the toner dust firmly to the drum surface.
Old printers charge their working surfaces with high voltages using devices called "corotrons" - corona wires held above a grid. The corona assembly is fed with a very high voltage (perhaps 4000 volts) but this is strapped down to something lower by a zener diode controlling the wire.
High voltages can break the bond between molecules of oxygen in the air. Single oxygen atoms are very unstable and will immediately link to O2 to create ozone - O3. Ozone is unstable and will give up one of its molecules quite readily to organic material like human tissue. Unfortunately ozone does this rather too energetically, not only is it poisonous, it is also thought to damage genetic material and so to be carcinogenic.
A high voltage in a laser printer will create a strong bond between toner and photoconductor or toner and paper, but will also create ozone. Printers generally need some air-flow, otherwise heat from the electronics and the fuser will become a problem. The air-flow also carries the ozone. Printer designers place an activated carbon air-filter next to the extractor fan and this mops up the greater part of the ozone.
The ozone filter gets contaminated with dust and grease and this reduces its effectiveness, so after some time it has to be replaced. Unfortunately printer designers often place the filter in inconvenient places, so the filter might not be changed as often as it ought to be.
High voltages also attract dust to the corona wires and this interferes with their operation, so the wires need periodic cleaning.
Worries about ozone mostly apply to old printers and perhaps to very fast photocopiers and printers.
Printer designers have largely succeeded in engineering high voltages out of ordinary workplace printers, using rubber voltage transfer rollers. Some ozone is almost inevitable, so it may not be advisable to have a printer confined in a very small and ill-ventilated space. There is generally no need for ozone filters in recent designs.
Ventilation is a point worth stressing. Copiers and big workgroup printers are a shared resource and office managers often love to put them in a little room where they don't disturb anyone, access can be controlled and they can be properly supervised. If it's a stationery closet or something like that then install an extractor fan.
If you can smell ozone then the levels are too high and either the machine needs maintenance or ventilation needs to be improved.
Carbon Monoxide
There are claims that the action of the fuser on the toner creates a significant amount of carbon monoxide. Given the temperature involved which is well below the combustion point this is surprising. It would reinforce the general recommendation that printers be used in well ventilated spaces.
Silicone Oil.
Most photocopiers and older printers avoid dirty print and paper jams in the fuser by coating the rollers with a very thin layer of silicone oil.
Some older copiers used a substantial amount of oil - they often had a feed-reservoir for it near the fuser and the entire fuser end of the machine was dripping with it. Engineers got a significant exposure. Users were exposed as well. Some of the oil inevitably transfers to the paper. This was the reason why it was often difficult to write with biro on photocopies - the oil made the biro tip slip.
Recent laser printers use little or no silicone oil. By the mid 1990s the fuser typically had a pad which was swapped with every new toner - so it had a life of about 10,000 copies.
In recent printers the fuser is usually a teflon sheet moving over a ceramic heater and there is no oil.
During the 1990s a controversy developed about silicone as a health hazard. It was alleged that breast implants made by Dow Corning leaked and caused connective tissue disorders. The implants did leak and Dow Corning settled several class actions and declared bankruptcy. Later evidence suggested that silicone was not a cause of connective tissue disease. In skin contact and in the quantities used there doesn't seem to be a hazard.
Summary.
Modern printer designs, especially the smaller devices produce little or no ozone. The fuser is still melting plastic into paper and there is usually a distinct odour in an ill ventilated room where a printer has been working hard for some time. Most of the odour is probably printer lubricant, some will be VOCs from the toner, some may be airborne particles.
Don't make heavy use of a laser printer in a very ill ventilated space. Inevitably something that exposes high voltages and heats plastic to melting point presents some sort of potential hazard. The hazards from laser printers don't seem to be greater than those of a TV or any other ink-using machine but there remains some uncertainty.
References:
The London Hazards Centre Factsheet--
© Graham Huskinson 2010
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