H‑1 Electrical Hazards
H-1. ELECTRICAL HAZARDS
Most people are aware of electrical hazards in general. It is worth pointing out, however, that the laboratory environment amplifies the hazards somewhat: voltages in some laboratory apparatus are frequently higher than normally encountered (although the familiar 115 volts can be lethal with a good ground and wet hands). Metal desk tops, fume hoods and fixtures provide body grounds; water lines and spills can furnish conduction paths. Always disconnect apparatus from the electric supply before tinkering; use insulating gloves and insulated tools where appropriate. In certain electrical equipment be sure to discharge the capacitors before beginning work on the apparatus. Do not defeat the purpose of interlocks, fuses, or circuit breakers designed to protect supply lines, equipment, and people. "One hand in the pocket" is good insurance when electric shock is a possibility, and should be adhered to rigidly when throwing open‑type switches, removing leads from terminal boards, pulling plug leads from a distribution board, operating line‑power rheostats, etc. Always be sure your hands are dry. Ground exposed parts of all electrical apparatus where possible. Beware of single‑pole double‑throw switches which may leave part of an apparatus at an AC potential with respect to ground when the switch is off. According to the present electrical codes, only three prong plugs where the third prong is used for grounding are legal. Be sure that power cords, insulators, and ground connections are in good condition, and that the fuses used are of the appropriate rating. Avoid temporary wiring. Be very careful with electrical apparatus that may be wet, especially heating mantles and devices with exposed conductors. Only use apparatus or equipment which has a UL, CSA or Ontario Hydro label on it.
A word of caution about variacs or variable voltage transformers ‑- VARIACS DONOT ISOLATE YOU FROM THE LINE VOLTAGE. Even a variac set at a low output voltage still has the full line voltage to ground. Any questions regarding the electrical safety of any apparatus or set‑up should be referred to the departmental electrical technician. Maintenance and repairs should only be undertaken by qualified persons. Repairs to electrical equipment are carried out only by Departmental technicians when authorized by the appropriate faculty member. Very high voltages carry special hazards. In dry air a spark can jump approximately one centimeter for each 10,000 volts; this distance is greater in wet air or with sharp‑pointed electrodes. X‑rays may be a significant hazard when the voltage exceeds about 15,000 volts; this hazard can be present in oscilloscopes, electron microscopes, and electron diffraction apparatus as well as from high voltage rectifier tubes.
H‑2. RADIATION HAZARDS
Aside from radioactivity, with which we will not deal further here, injurious radiation in the laboratory comes under four principal headings: X‑ray, ultraviolet, laser, and radio‑frequency radiation.
X‑rays are used by chemists primarily in crystallographic diffraction work. Such work should be performed only by persons who have been adequately trained in the procedures and precautions, or under the immediate and responsible direction of such a person. An X‑ray generator and diffraction apparatus must be shielded with 2 mm of lead so that no direct radiation escapes into the room from either of them or the junction between them, and so that scattered radiation arising from the junction, the crystal, and the beam stop are at the lowest possible levels. Every diffraction experiment should be monitored with a counter‑type survey meter and it should be ascertained that the radiation level at all points around the apparatus is no more than 10 millirem/hour, and preferably less.
The allowed occupational dosage of whole‑body X‑radiation is 300 mrem/week (but no more than 5000/year); hands or forearms are allowed several times that output. CAUTION: The intensity of the X‑ray beam as it exits the window may be 104 rem or 107mrem/min! Local exposures of more than 1000 rem (easily obtained on the fingers when working on the apparatus with a tube port open) can produce serious skin burns; much smaller exposures can probably cause eye cataracts. A whole‑body one‑time exposure of 500 rem of penetrating X‑rays is usually fatal, but difficult to imagine as coming from a diffraction X‑ray generator since the beam is somewhat confined; also, the principal component of X‑rays used in diffraction penetrates only a few millimeters into the body.
An insidious hazard with some X‑ray units is back‑conduction of a gassy rectifier circuit; this can result in radiation of twice the nominal maximum energy of the beam from the X‑ray tube, and of greater penetrating power. Adequate shielding of the transformer‑rectifier unit (3 mm of lead), good periodic maintenance, and routine monitoring of the radiation background are essential. Walls, ceilings, and floors do not provide reliable isolation from crystallographic X‑rays. At the least, the equivalent to 13cm (about five inches) of solid concrete is needed to provide adequate protection; wooden doors and plasterboard partitions offer essentially no protection from X‑rays.
The laboratory and all personnel who enter it must be equipped with film badges mounted at appropriate positions to detect escaping radiation. Before the X‑ray unit is turned on answer the question: are all X‑ray ports adequately covered? Before leaving the room, be sure that no mechanical malfunction of the diffraction camera can possibly lead to radiation hazard during the run (by slippage of a shield, displacement of a collimator, etc.). A woman in the first few weeks of pregnancy should avoid exposures to X‑rays as the fetus is extraordinarily sensitive to injury that is likely to result in subsequent malformation.
H‑2.2. ULTRAVIOLET RADIATION
Ultraviolet radiation (such as is used in spectroscopic and fluorescence experiments) can provide skin burns (akin to sunburn) and, more especially eye damage (particularly cornea and lens). Eyeglasses provide some protection; special goggles (with side protection) are better. In any case, careful attention should be given to shielding the experiment to prevent the escape of any direct beam or any significant amount of scattered radiation.
H‑2.3. LASER RADIATION
Laser radiation is potentially dangerous to the eyes (retina) because of the high energy content available in the laser beam and because, being coherent and parallel, the beam can be focused to provide exceedingly high local intensities. Adequate shielding must be provided. Eyeglasses, of course, give no protection from lasers emitting in the visible range. Glasses which block light of the frequency being emitted by the laser should be used. Just because glasses have a dark tint does not mean that they will block light of a given frequency.
H‑2.4. RADIO-FREQUENCY RADIATION
Radio‑frequency power such as is used in induction melting of alloys and certain microwave applications can be absorbed deeply in human tissues, "cooking" them quickly and causing deep burns. Shield induction units as well as possible; keep your body out of a microwave beam.
Eye damage can also be produced by flash tubes, very bright sparks, and arcs.
H‑2.6. STRONG MAGNETIC FIELDS
The high‑field nmr magnets have strong, stray magnetic fields which are of potential danger to wearers of pacemakers. These fields are also felt in rooms immediately above and below the magnet. In addition, users of these instruments should be aware that the magnetic strips on credit cards and the information stored on floppy disks can be wiped clean by these stray fields. Even "anti‑magnetic" watches and especially quartz watches are affected by such fields.
A centrifuge contains a high-speed rotor which is under considerable stress owing to centrifugal force. In a well‑designed centrifuge properly operated, these stresses are well below those required to rupture the rotor. However, abuse of this apparatus can result in the rotor being more susceptible to rupture at a given stress; moreover, the stresses can be considerably increased by vibrations arising from imbalance. When the rotor does rupture, its fragments become dangerous high‑speed projectiles similar to bullets or shrapnel which can in some circumstances rip through the steel outer jacket, through partitions between rooms and then through human flesh. A high‑speed centrifuge should have an adequate barrier against such a possibility; for an "ultracentrifuge" this might be a reinforced concrete wall about a foot thick. The lid of the centrifuge must be closed whenever the centrifuge is in operation.
Besides providing an adequate barrier against bursting, it is important to guard against imbalance that is the principal cause of rotor failure. Each centrifuge tube containing a sample must be balanced with another of this same gross weight (or rather, moment of inertia) diametrically opposite. Be careful that imbalance does not occur during the run owing to differential evaporation of solvent. This can easily happen when two liquids of different vapour pressure are present diametrically opposite, or when a concentrated solution is initially balanced with the same weight of pure solvent. It is best to balance each sample with another identical to it.
While the centrifuge is operating, and especially while it is approaching its operating speed, be alert for unusual noises or other evidence of excessive vibration; turn it off if any should arise. Never attempt to stop a centrifuge by mechanical interference; never put your hands into the centrifuge until the rotor is absolutely stationary.