H‑1
Electrical Hazards
H‑2 Radiation
H‑3 Centrifuges
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.
H‑2.1. X‑RAYS
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.
H‑3. CENTRIFUGES
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.