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72 PHYSICALSECURITYwhen problems arise. Ideally, HVAC units should be in-
stalled in pairs, with each unit being able to carry the
load of the other should it malfunction.
Although some information is only of transitory value,
other data, such as official records of births, deaths, mar-
riages, and transfers of property ownership, should be
kept in perpetuity. Standards for long-term preservation
of data stored in magnetic or optical format are far stricter
than guidelines for ordinary usage. As a sample, for preser-
vation, the prescribed allowable temperature variation in
24 hours is a mere± 1 ◦C(2◦F). See International Advi-
sory Committee for the UNESCO Memory of the World
Programme (2000) for detailed preservation guidelines.
One such guideline is that magnetic media, both tapes
and disks, be stored in an upright orientation (i.e., with
their axes of rotation horizontal). The exclusion of light
is important for extending the useful life of optical media
incorporating dyes (writeable discs). All media should be
stored in containers that will not chemically interact with
the media. Projected life spans for properly archived me-
dia are considered to be 5–10 years for floppy diskettes,
10–30 years for magnetic tapes, and 20–30 years for op-
tical media. These estimates are conservative to ensure
creation of a new copy before degradation is sufficient to
invert any bits.
For optical media, life expectancies are extrapolated
from accelerated aging tests based on assumptions and
end-of-life criteria that may be invalid. Numerous factors
influence longevity. Write-once formats have greater life
expectancies than rewriteable formats. The bit-encoding
dye phthalocyanine (appearing gold or yellowish green)
is less susceptible than cyanine (green or blue-green) to
damage from light after data has been written; yet manu-
facturers’ claimed life expectancies of up to 300 years are
not universally accepted. What appears to be a major de-
terminer of longevity is the original quality of the stored
data. This in turn depends on the quality of the blank disc,
the quality of the machine writing the data, and speed at
which data was written. Hartke (2001) gives an enlighten-
ing look at the complexities of this issue.
All archived data of critical importance should be sam-
pled periodically and backed upwell beforethe rate of
correctable errors indicates that data might be unrecov-
erable at the next sampling. Even physically perfect data
has been effectively lost because it outlived the software or
hardware needed to read it. Therefore, before its storage
format becomes obsolete, the data must be converted to
an actively supported format.
There are devices or consumable products for clean-
ing every type of storage medium and every part of a
computer or peripheral device. Backup tapes that are fre-
quently overwritten should be periodically removed from
service to be tested on atape certifier,which writes sample
data to the tape and reads it back to detect any errors;
some models incorporate selective cleaning as an option.
Read-write heads for magnetic media typically need to be
cleaned far more often than the medium that moves by
them. For optical media, clean discs are usually the con-
cern. Compressed air should not be used; the resulting
drop in temperature produces athermal shock(rapid tem-
perature change) for the disc. If the problem is scratches
rather than dirt, polishing may be required.Keeping a computing area free of foreign particles is
a multifaceted task. Air filters should remove fine dust
particles because outdoor dust is brought in on clothes
and shoes. Filters must be cleaned or replaced on a reg-
ular schedule. Periodically, air-heating equipment should
be turned on briefly even when not needed. This is to in-
crementally burn off dust that would otherwise accumu-
late and be converted to an appreciable amount of smoke
when the equipment is activated for the first time after
a long period of disuse. Vacuuming of rooms and equip-
ment should also involve filters. Food, drink, and tobacco
products should be banned from the computing area.
Water detectors should be placed above and below
a raised floor to monitor the rise of water. An auto-
matic power shutdown should be triggered by a sensor
that is lower than the lowest energized wire. Degaussers
and any other equipment that produces strong magnetic
fields should be kept in a room separate from any me-
dia not scheduled to be erased. Although the intensity of
most magnetic fields decreases rapidly with distance, it is
very difficult to shield against them. Likewise, computers
should be kept away from sources of vibrations, including
printers. If this cannot be arranged, vibration-absorbing
mats can be placed under the computer or the offending
device.Health and Safety Issues
The humans in the computing environment have addi-
tional needs. Some general health issues that may arise
aresick building syndrome(symptoms arising from toxic
mold) andLegionnaire’s disease(a form of pneumonia
transmitted via mist and sometimes associated with large
air conditioning systems). Human-friendly appointments
pertinent to a computing environment include the fol-
lowing:- special keyboards or attachments that optimize wrist
placement; - comfortable, adjustable chairs that properly support
backs; and - special lighting, monitor hoods, or screen coverings
that reduce glare and, therefore, eyestrain.
There is currently no consensus on the long-term ef-
fects ofextremely low-frequency(ELF) emissions (below
300 Hz),magnetic fields emitted by a variety of devices,
including high-tension lines and cathode ray tube moni-
tors (but not LCD displays). Laboratory tests with animals
have found that prolonged exposure to ELF fields may
cause cancer or reproductive problems. Studies of preg-
nant CRT users have produced conflicting data. Pending
conclusive evidence, some recommend keeping 60 cen-
timeters (2 feet) away from such monitors, which may
not be practical. There are similar concerns and uncer-
tainty with regard to cellular phones. It is known that
people with pacemakers should avoid devices creating
strong magnetic fields, such as degaussers. Although the
World Health Organization acknowledges the need for
continued research in certain areas, its latest position
is that there is no evidence of health risks associated
with EMF exposuresbelowthe levels set forth by the