The URL of this page is http://www.bio.cam.ac.uk/nmr/safety.html
All users of the National 800 MHz Facility will be asked to sign a safety
form stating that they have read and understood the departmental safety
information booklet and the risk assessment (below).
Risk assessment for NMR rooms belonging to the Dept. of Biochemistry and
CCMR NMR Facility
The main risks come from the high magnetic fields, the handling of cryogens
and high electrical voltages/RF sources. In general, the risks are minimised
by limiting access to the NMR rooms to only the NMR staff and users of the
instruments and by limiting any hardware modifications/maintenance to the
NMR staff (or exceptionally very experienced users) who have the requisite
technical understanding and training.
The NMR staff comprises Drs E. Laue, D. Nietlispach, K. Stott and Mr M. Sparkes
(Technician).
Electrical/RF
Risks are similar to those encountered in the use/maintenance of other
laboratory equipment and are minimised by restricting any
modification/maintenance of the equipment to the NMR staff (or
exceptionally very experienced users) in consultation with the manufacturer.
Only carbon dioxide fire extinguishers should be used to avoid equipment
damage and exceptional care is needed to ensure that fire extinguishers are
not used near the magnet cryostat. In case of serious flooding, or in
other situations where there is risk of electrocution, the equipment
circuit breakers should be turned off in the plant room on level 00.
(No water supplies are actually used in the NMR rooms).
Cryogens
The cryogens used are liquid N2 and liquid He:
Temperature: N2: -196 and He: -269 deg. C
Colour: none
Toxicity: very low
Fire hazard: non combustible
Volume Expansion (from normal boiling point to room temp.): ca. 700x
The main risks are of burns when handling cryogens and of asphyxiation if a
magnet quenches. These are minimized by only allowing very experienced
technicians to fill the magnets with liquid nitrogen and (at present)
by contracting out the filling with liquid helium to Oxford Instruments.
The magnet cryostats are only filled using stainless steel transfer lines
to reduce risk of rupture.
At least two staff must be present during refilling and
appropriate safety clothing must be worn (gloves and eye protection).
Refills must be continously attended.
It is particularly important that the person filling the magnet, once
trained, should do so on a very regular basis so as to be very familar with
the required routine. Magnet quenches (the rapid release of gaseous cryogens
from the cryostat into the room) should trigger the fire alarm and
thus open the smoke vents thereby
preventing any risk of asphyxiation due to the large volume expansion.
In the event of a quench personnel should evacuate the area (a quench
warranting evacuation would be obvious by the noise of the escaping gas and
clouds of vapor).
Access to the NMR rooms is strictly limited to the NMR staff during
refills and any major maintenance. During other periods access is limited to
a known set of users via combination locks. For the 800MHz NMR magnet system,
which is standing in a pit, there is a potential danger of asphyxiation during
N2 fills because the density of N2 is higher than of O2. Access to the pit is
therefore prohibited during refilling of the magnet.
The magnet cryostats continuously expel a small quantity of gaseous He and N2
into the air. This does not present a hazard since during everyday use the
air is constantly changed in the NMR rooms by the air-conditioning system.
Any drop in the oxygen content of the air can be detected by an oxygen monitoring
system (these are due to be installed soon).
A site-specific operating procedure is available from the NMR staff for topping
up of liquid cryogens.
High Magnetic Fields
Large attractive forces are exerted on magnetic materials or equipment
bought in close proximity to the NMR magnet systems which are always at
field. The force may become large enough to move tools or equipment
uncontrollably towards the magnet system and the closer to the magnet
system the larger the force.
This is probably the main risk because it is one with which most people are
unfamilar. Risks come from release of any steel items/tools or equipment
which are bought near the magnets. Even belt buckles, steel tipped shoes
etc. may be strongly attracted to a magnet. This might result in damage to
the magnets/probes
(possibly resulting in a quench) or serious injury or death to personnel
working near or under a magnet. The latter is probably the most serious
risk, as staff and users often need to work under the magnet during its
normal operation.
These risks are mimimised by preventing access to the NMR rooms by anyone
other than the NMR staff and trained users. Anyone else needing to enter
the NMR rooms can only do so in the presence of one of the NMR staff.
Combination locks are reset regularly to prevent their codes becoming known
by others.
It is necessary to use steel tools for maintenance and repair of the
consoles, but such work should only be done by the NMR staff (or engineers
from the manufacturers) and users are not allowed near the magnet during
such work. Where possible non-magnetic tools are used on the magnets
themselves.
Medical electronic implants such as e.g. cardiac pacemakers may be affected
by static or changing magnetic fields (not all pacemakers respond the same
way). Medical implants e.g. clips, protheses which may contain ferromagnetic
materials would be subject to strong attractive forces near to the NMR
magnet system. All people with such implants are excluded from the NMR
rooms and appropriate warning signs are displayed.
Recent publications suggest that long term cumulative large exposures to
oscillating magnetic fields (60 Hz) may be associated with increased
incidence of brain cancer in power industry workers. The cumulative doses
for an effect were large, and represent no hazard outside the 5 gauss
safety line normally used. Reasonable caution in avoiding lengthy exposure
to higher fields seems prudent, none the less. Our current advice is that
NMR staff and researchers should spend no longer than reasonably necessary
within the 5 gauss line for sample changing and adjustments. No other
equipment unrelated to the NMR system is placed within the 5 gauss line,
which should be contained in the NMR rooms themselves. The NMR magnets will
cause disturbance to VDUs etc. in nearby laboratories on Levels 0 and 1
(eg. where the stray field is 1-2 gauss). In many cases Users of nearby
laboratories will need to screen their VDUs and advise etc. is available
from the NMR staff. There should, however, be no safety implications.
see: David A. Savitz and Dana P. Loomis (1995). "Magnetic field exposure in
relation to leukemia and brain cancer mortality among electric utility
workers." Am. J. Epidemiology 141(1), 123-134.
Magnetic fields may permanently damage watches, calculators and certain
types of credit cards. These items need to be kept well away from the
magnets.
Video Display Terminals (VDTs)
Extensive radiation measurements and health data do not indicate that these
units present a health risk. Most of the symptoms related to the use of
computer terminals are related to strain and discomfort that can be
corrected by ergonomic measures. The potential for eye strain can be
reduced by the use of non-glare screens, fitting a filter to the VDU, or by
improving the lighting conditions. Users are strongly advised to consult
the Universities Code of Practice for the safe use of VDUs (available from
the NMR staff).
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Dr Daniel Nietlispach