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Scand
j.
work environ
health 5 1979
280-285
emper ture
ch nges
in cont ct
lenses
connection
with
r di tion from infr red
he ters
by
PER LOVSUND,
M.SC.E.E.,l SVEN ERIK
G.
NILSSON, M.D.,
Ph.D.,2
and
P. AKE OBERG, Ph.D.I
LOVSUND,
P. ,
NILSSON, S. E. G. and OBERG, P.
A. Temperature
changes
in
contact
lenses
in
connection
with radiation from infrared heaters. Scand j. work
environ health 5 1979) 280-285.
number
of
reports have appeared over the
past
few years with
warnings about
th e
wearing
of contac t l ense s
i n cert ain t rades
involving
exposure
to
arc
flash.
In view
of
these reports and i n light
of knowledge
on th e
marked
absorption by contact lenses, within the infrared R) region, tem
perature changes
were
measured
in
soft contact lenses under radiation from IR
heaters
used, for example,
i n t he motor indust ry
for
drying
paint.
The
lenses
were
tested while free-hanging and when applied to rabbit
eyes.
Great increases
in
tem
perature
were
noted with one of th e
heaters
at a dis tance corresponding to safe.
During 10
min
of
exposure the tempera ture
of a
free-hanging
lens rose from
21
to
59°C,
whereas the temperature in th e surrounding air increased
only
from 26 to
30°C.
The
final temperature
of
the lens was
thus
29°C
higher
t han tha t
of
the
air.
In lenses applied
to rabbit
eyes
th e temperature
rose
within
only 0.5, 1.0, and 1.5
min from approximately
33°C
to about
44, 49,
and
51°C,
respectively;
the air tem
perature rose f rom 25 to only 28°C. In other
words,
the u lt imate
lens
temperature
was
23°C
higher than
the
ambient ai r temperature.
In t he rabbi t experiments most
of the
lenses
dried out
completely.
There
would
thus
seem
to be
considerable
risk
of
contact
lenses
dry ing and
becoming
adherent
to and damaging th e corneal sur
face among
workers
exposed to power fu l
radiation
from
IR rad ia tors
IR heaters ,
unless
they use
e ff ic ien t eye protector s.
IR heaters appear to be
associated
with
greater
hazards
than
arc flashes,
since
there is
no
warning from powerful
visible light and because
they
are capable
of
causing a very rap id increase
in
tem
perature.
Key
words
corneal damage,
infrared
heaters,
radiation
hazards, sof t contact lenses,
temperature
changes,
worker
safety.
has been a ss er te d
on
several occasions
in
t he l it er at ur e that welding
arcs and
other h igh-energy a rc s may constitute a
serious
hazard
for
wearers of
contact
Department
of Biomedical Engineering, Uni
versity of
Linkoping, Sweden.
2
Department
of Ophthalmology,
University
of
Link6ping,
Sweden.
Reprint requests
to: Prof. S. E. G. Nilsson,
Department
of Ophthalmology,
University
Hos
pital, Un iver sity of Link6ping,
S-581
85 Lin
k6ping,
Sweden.
0355-3140/79/030280-6
lenses
1,
4, 7, 9,
10, 13).
The case that set
off
th e
discussion
was that of
a
shipyard
wo rker w it h
con tact l ense s who developed
corneal
lesions after exposure
to a
power
ful
f lash occurr ing
when
a circuit
breaker
arced durin g th e connect ion o f a welding
cable. He
was
wear ing s af et y glasses
at
the time.
has since
been
alleged that
the
injury
was due
to
17-18
h
of con
tinuous
wearing
of
the lenses
8,
10,
11,
12)
and
that
the original
report
was noth
ing
but
an indus tr ia l a troc ity s to ry
12).
No
e xp erime nta l in ves tig ation s o n
the
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problem have been carried out
to
our
knowledge however,
and
existing
reports
offer
too flimsy a bas is for recommenda
tions
to contact lens
wearers exposed daily
to
electric
arcs
or o ther
radiation.
there
fore seems a matter of interest to elucidate
the question.
This investigation
is
a
direct continua
tion of studies on the t ransmit tance and
absorption properties of certain contact
lens
materials
3
and on temperature
changes
in
contact lenses during
radiation
from certain welding processes 2
that
we
have car ried out.
We
have
shown that
marked
absorption occurs
in the infrared
IR) region 3 .
The
present
work
was
therefore undertaken
to
investigate th e
temperature
changes that take
place in
contact
lenses on exposure
to
radiation
f rom two dif feren t
IR
heaters.
IR heaters are used,
fo r
example,
in
th e
motor
industry and
at workshops
for d ry
ing paint on cars.
is
suspected
that
IR
radiation
may cause
damage
to
th e eyes
radiation cataract). The Swedish
National
Board
of Occupational Safety and Health
has therefore analyzed work
conditions
in
connection
with IR
heaters
and has
also
investigated th e emission from 10 common
heaters
6).
The
heaters
were examined
with
regard to
spectral
distribution, ra
diance
at
different
distances,
source area,
luminance, and other factors.
MATERIAL AND METHODS
Temperature re or ings
Copper-constantan
T) thermocouples
High
Temperature
Instruments
Corpora
t ion, U.S.A.)
were
used. The tips were
flattened
to
approximately 80 / The
tem
perature
was
read from
a five-channel
digital indicator oric, U.S.A.)
with
a
res
olution
of 0.1°C and with built-in lineari
zation and compensation for the cold junc
tion.
The following control exper iment was
done to establish
whether a false
rise
temperature may
occur as a result of
IR
radiation absorption
by the naked
thermo
couple.
Two
identical
thermocouples
were
mounted
at
th e same level and 0.2 m
apart
at
a
dis tance of
0.5
m
from one
of
th e IR
heaters dcoat). A powerful air
current
was
fanned
over them from one side. The
same
air
temperature was recorded
by
both thermocouples. When one of them
was screened from th e
IR
radiation
no
change
in temperature
was noted
in either
thermocouple. Thus, no false
temperature
increase
from
the absorption of IR radia
tion
by the naked
thermocouple
was found.
nfrared heaters
IR
radiation
was
generated by two diffe r
ent
heaters,
Elektrofem
IF-03, 6 kW)
and
Adcoat
AB
IR 2200, 2.2 kW).
Their main
specifications
are summarized in
table
1
and
fig.
1.
The
safe
distances calculated
for the
hea te rs a re taken
from the
recom
mendations of the Swedish National Board
of
Occupational
Safety and
Health 6),
and
they
are based
on
th e estimated
risk
of
cataract caused by absorption of near IR
radiation by
the lens
of
the eye
5).
easurement of temperature in
free-
hanging
contact tenses
Soft lenses of th e HEMA hydroxyethy
methacrylate) type
were
used.
The
ther
mocouple was placed
at
th e center of
the
lens with the probe tip
inside
th e lens. The
lens was
thoroughly wet with
physiological
saline
at
the s ta rt of
each
recording. Two
makes of lens were used, Weicon 15.50
D
or
Soflens
17.00 D,
both
of which
have a water content of 39 and a lens
thickness
in the wet state) of approxi
mately
0.5
mm
at
th e point where
the
temperature
was recorded. A second ther
mocouple placed in
front
of th e
lens
was
used
to
record
ai r
t empera ture . The
radia
tion f rom
the IR heater
was direc ted
to
wards th e front of the lens.
easurement of temperature in contact
tenses
applied
to
rabbit
eyes
The exper iments with rabbits were done
so
that
th e
true
conditions
at
a place of
work could
be reproduced as
closely
as
possible. Soft, HEMA-type lenses were
used,
applied
to
the
eyes of adult,
pig-
281
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Table 1.
Specifications
of
t he inf ra red hea te rs
[from
Ruth et al . 6 ].
Infrared
Spectral
Approx.
Power
Luminance
Safe
heater
region radiation max.
kW
cd/m
2
distance
nm
nm
m
Elektrofem
1,000 -
>
3,200 2,500
6.0 0
0.5
IF-03
Adcoat AB,
750 -
>
3,200
3,300
2.2
40 1.0
IR
2200
500 1000 1500 2000 2500 3000
Wavelength
nm
ElEKTROFEM
kW
100
500 1000 1500 2000 2500 3000
Wavelength nm
O O T kW
ig
1 Spectral dis tr ibut ion curves for
the
two
infrared
heaters [from
Ruth et al . 6 ].
mented
rabbits. The animals
were
awake
and
unanesthetized.
In these experiments
it
proved impos
s ib le to retain the thermocouple in a
single
lens,
as it became
dislocated by the least
eye
movement. A double lens one Hydron
- 1.75 D and
one
Soflens + 3.50 D was
therefore
constructed in which
th e radii
of
curvature of the
inner
lens
matched ex
actly
those
of
the cornea
and th e
posterior
surface of the
outer
lens. The lenses
were
glued together
at
the
extreme periphery,
and
the
thermocouple,
flexed to fit,
was
introduced between the wet lenses, care
being
taken
to exc lude
air bubbles. The
wet
combination
was 0.6 mm thick at the
measuring point. In
these
experiments
only the Elektrofem
heater
was used.
Changes
in air temperature were
record
ed
with a thermocouple in front of the
lens. The animal was
arranged
so that the
optical axis
of the
eye
under
investigation
was directed towards the IR heater. The
lenses
were rewet
between each
experi
ment.
282
RESULTS
emper ture
ch nges in free h nging con-
t c t lenses
Temperature
changes
in the lenses and the
surrounding air during 10 min of con
tinuous radiation from each of the two
IR
heaters at a
distance
of 0 5 m are shown
in table
2
No significant difference was
found between the two
types
of lens, and
the results
are
therefore given
as
the
means of the two. In absolute figures
the
lens
temperature
increased
in 1 3 and 10
min
from 21°C to 42, 50, and 59°C, re
spectively, with the Elektrofem heater.
With the
Adcoat
the lens temperature
rose
from
20
to 45°C in 10 min.
The air tem
perature
rose
from
26
to
30°C and
from
25 to 28°C for the two heaters, respective
ly ;
in
other
words
the
final
temperatures
in the lenses were 29
and
17°C higher
than
in
the surrounding air. With Elektro
fern
the
distance tal l ied with
the
safe
dis
tance.
For Adcoat
the
safe distance is
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1 m,
however, and with radiation
at
this
distance th e lens temperature
did not
in
crease
more
than that of th e surrounding
air.
emper ture ch nges in ont t lenses
pplied
to
r bbit
eyes
was found that
in the
conscious
animal
th e unanesthetized
eye,
equipped with
a
double lens,
was often shut o r almost shut
when
exposed to
th e IR heater.
When
th e
eye was completely shut ,
th e
increase
in
temperature
was
halted o r
reversed. The
results are therefore
divided
into
record
ings made with
th e
eye open and with
an
approximately 3-mm palpeb ral fissure.
The dis tance was
0.5 m,
which corresponds
to
the
safe
distance for
th e
Elektrofem
heater.
The
results
are
given in
table
3
and
fig.
2 Exposures of
0.5, 1.0
and
1.5
min with the eye fully
open
caused th e
lens temperature
to increase from about
33°C
to
approximately 44, 49, and 51°C,
respectively. Because
th e
air
temperature
rose only f rom 25
to 28°C,
the increases
in the lenses
were 16, 21, and 23°C
greater
than in the surrounding air. With
an
ap
proximate ly 3-mm palpebral fissure and
an exposure t ime of
1.5
min th e lens tem
perature
increased
to only
44°C average .
In al l th e experiments in which
th e
eye
was kept
open,
and
also
in several in
which th e
palpebral fissure was slight ly
smaller, th e lenses dried completely and,
as
a
rule,
also
became deformed and fell
out of the
eye.
In
a
few
cases
th e
lenses
became
adherent
to
th e
corneal surface.
In one exper iment the tempera tu re of
th e naked
corneal
su rface was measured
during radiation from the Elektrofem
heater
at
a d istance of 0.5 m.
The
tem
perature of th e corneal surface
a lso rose,
and the
increase was pract ical ly ident ical
to
that recorded
in
th e contact
lenses.
DISCUSSION
The two
IR
heaters tested
emit
radiation
within
roughly th e same
region of the
spectrum. The power is
near ly three times
greater
in Elektrofem 6
kW
than in
Adcoat
2.2 kW ,
however.
Of
the heaters
tes ted by the
Swedish National Board of
Occupational Safety and Health, Elektro
fem had the greatest power, whereas Ad
coat was among
those with
the
lowest
power. Because both generate
very l it tl e
visible
light,
there
is
only
a
limited warn
ing of
th e
radiation,
and prolonged expo
sure
may occur. A
worker with contact
lenses therefore
runs
th e risk that the
l enses become overhea ted, dry out, and
become
adherent
to
the
corneal
surface.
The cornea
itself
and
it s
tear
film
also
show
marked
absorption in the IR region
and consequently also
an increase
in
tem
perature.
Without contact lenses, however,
the th in
tear film
is
probably
maintained
by blinking. On
th e other
hand, it
seems
ble
2.
Increase in temperature
0C)
in wet, free-hanging contact lenses
after
10
min of radiation: A comparison of two infrared heaters.
Infrared heater
Elektrofem N =
2
Adcoat
N
=
3
Lens temperature
Initial
21 20
Final
59 45
Increase
38
25
Air temperature
Initial
26
25
Final
30 28
Increase
4 3
Difference between
in l
lens and
air
temperatures
29
17
283
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Table 3. Increase
in
temperature
0C
in contact lenses applied to rabbit eyes.
Eye fully open throughout experiment
Experimental
time 1 min
=
3
Experimental
time 1 5 min
= 1
Palpebral fissure
about 3 mm wide,
experimental time
1 5 min
N
= 5
Lens temperature
Initial
33
32
33
Final
49
51
44
Increase
16 19
Air temperature
Initial
25
25
26
Final 28 28 28
Increase
3 3
2
Difference between final
+
21
+
23
+ 16
lens and air temperatures
30
55
T M
sec >
that water-containing
contact lenses
carry
a considerable risk
of
drying out,
and
therefore they may become adherent to
th e corneal surface and cause
epithelial
lesions.
Thus
t he hazard is not primarily
due
to th e rise in temperature but to the
adherence
of
the contact
lens.
No
epithelial
lesions
caused
by adherent
lenses were found in th e rabbit
experi
ments. The rabbit
cornea
is
much more
resistant to mechanica l
trauma than
th e
human counterpart , however. Also, con
tact
lenses
ar e
fitted
with much greater
precision
in
practice in man than they
were
in these
animal experiments.
Fur
thermore the weight of th e thermocouple
constituted a force tending to lift the lens
from
the eye. I t may
therefore
be assumed
that
the
better fi t of
and the
lack of
ther
mocouple force
on
th e
human
lens
will
contribute
to a
more
complete adhesion
of
th e lens.
Workers
should therefore
be
ad
vised
against
wearing
contact
lenses while
operating
long-wave
IR radiators unless
they also wear
safety
glasses that
absorb
the IR radiation.
Some other
IR
heaters in
use produce
radiation
of considerably
longer
wave
lengths than the
two tes ted
in
this
study.
The hazards with these are undoubtedly
even greater ,
as
th e emission
lies
within a
region
that
is
absorbed
to
almost
100
0/ 0
by contact lenses 3 .
=5
In:t
0
0
.,.------+------
O::t
o
ig 2. Increase in temperature in contact
lenses applied to rabbit eyes. 0 0 and
=
open eyes,
--
=
palpebralfissure about
3
mm wide,
=
surround
ing air temperature
284
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ACKNOWLEDGMENTS
This
investigation
was supported by
the
Swedish Work
Environment
Fund
project
no. 76/174.
We wish to thank
Mr.
H. Lindh for fit
ting
the
contact lenses and
Ms. M. Old
back who
assisted in the animal exper i
ments.
REFERENCES
1. FOX S. L. Industrial and occupational
ophthalmology Charles C
Thomas
Spring
field,
IL
1973. 203 p.
2
LOVSUND, P., NILSSON S. E. G.,
LINDH
H.
and OBERG
P.
A Temperature chang
es in contact lenses in connection with ra
dia tion f rom welding
arcs.
Scand
j
work
environ
health
5 1979) 271-279.
3
NILSSON S. E. G., LOVSUND, P., OBERG,
P. A.
and
FLORDAHL L.-E.
The trans
mittance
and absorption properties of con-
Received
for publication: 6 November 1979
tact lenses. Scand j
work
environ
health
5 1979) 262-270.
4 NOVAK J.
F.
and
SAUL
R W. Contact
lenses in industry.
J.
occup
med
13 1971)
175-178.
5. RUTH W. A
method to eva luat e
occupa
tional hazards from infrared radiation.
Project report submitted
in part
fulfillment
of th e
requirements
for the
postgraduate
course
in ergonomics at th e
Loughborough
University
of Technology l i75. 156
p.
6 RUTH, W
LEVIN,
M and KNAVE, B.
rbetshygienisk
bedomning av infrarod-
stralare fOr
torkning
av biUack Under
sokningsrapport
AMMF
104/76). Arbetar
skyddsstyrelsen Stockholm 1976. 69
p.
7. Contact lens hazard.
Qual
eng 38
1974) 95.
8. Contact lens hazard. Qual
eng
38
1974) 150.
9
Contact
lenses
and arc
welders.
r
med
j
4 1974) 586.
10. Contact
lenses
and electrical
arcs.
Weld ins
res bul
15 1974)
43-44.
11. Flash and contact
lenses.
Occup
saf
health
5 1975) 11.
12. An
industrial
atrocity story.
Occup
saf
health
7 1977) 85.
13. Position statement -
Contact
lenses
in industry.
Ind
med
41
1972)
38-39.
285
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