Occup. Med. Vol. 46, No. 2, pp. 125-130,1996
Copyright © 1996 Rapid Science Publishers for SOM
Printed in Great Britain. All rights reserved
0962-7480/96
Neuropsychological symptoms
among tanker drivers exposed
to gasoline
M. Hakkola, M.-L. Honkasalo and P. Pulkkinen
Department of Public Health, PO Box 21 (Haartmaninkatu 3), 00014
University of Helsinki, Finland
The purpose of this study was to investigate the occurrence of neuropsychological
symptoms over periods of one week and one month among tanker drivers as related
to exposure to gasoline (methyl-tert-buthyl ether 10%). Milk delivery drivers acted as
controls. In addition to exposure to gasoline, age, chronic diseases, perceived
health, working time, work history in the occupation of driver and alcohol
consumption were scrutinized for their, associations with the symptoms. The target
group for the study consisted of 101 road.tanker drivers from three Finnish oil
companies all around Finland. The control group was 100 milk delivery drivers from
two milk companies from the same localities in Finland as the tanker drivers.
Standardized symptom interviews were conducted for all drivers. The differences
in the occurrences of neuropsychological symptoms between tanker drivers and
controls were not statistically significant. The age of the drivers, chronic diseases
and perceived health were connected to the occurrence of symptoms among
drivers. The results of this study do not exclude the possibility that vulnerable
groups exhibit an increased sensitivity to gasoline.
Occup. Med. Vol. 46,125-130,1996
Received 17 May 1995;accepted in final form 1 November 1995.
INTRODUCTION
Many chemicals in gasoline have acute and chronic
ill-effects. Tanker drivers are exposed to such gasoline
vapours at work. There are few studies concerning
symptoms connected to gasoline exposure.
Kumar, et al. performed neurobehavioural studies
on 90 petrol pump workers in India.1 Sixty-four control
subjects of similar age and socio-economic status were
also examined. No attempts were made to quantify
the level of exposure. The authors concluded that
immediate and delayed memory was significantly
affected in exposed workers. Intellectual capacity and
psychomotor learning ability were also significantly
affected.
In Sweden, Knave and Knave have studied the
neurological effects of the exposure to jet fuel.2 They
examined 29 workers, who had been exposed to jet
fuel vapours over a period at least 5 years. Thirteen
workers had been exposed daily or almost daily to
300-5,400 ppm jet fuel vapour. Most workers exam-
Correspondence and reprint requests to: Dr M. Hakkola, Department
of Public Health, PO Box 21 (Haartmaninkatu 3), 00014 University
of Helsinki, Finland.
ined complained of neurasthenic symptoms and had
lowered nerve conducting velocity. In another study
on 30 workers exposed to jet fuel during 2 years, no
changes in the central nervous system were observed
except for neurasthenic symptoms: anxiety and depression.3
Mohr, et al. conducted a study on the self-reported
symptoms of garage workers in the state of New Jersey
exposed to high and low MTBE concentration environments.4 Two hundred thirty-seven participants were
divided into low and high MTBE exposure. The participants were asked to indicate the frequency of
symptoms they had experienced over the last 30 days.
In addition workers were given identical preshift-postshift questionnaires and asked to rank any discomfort
they were experiencing at that time, using a list of
symptoms. No differences were found between the
groups in frequency of symptoms. Both groups felt
significantly worse by the end of work day, but there
were no differences between the groups across the
work shift.
Fiedler, et al. studied the response of sensitive groups
to MTBE.5 The purpose of the study was to assess
symptomatic responses among those persons known
to report sensitivities to very low-level chemical
exposures. Structured telephone interviews were con-
126
Occup. Med. Vol. 46, 1996
ducted for fourteen persons who had multiple chemical
sensitivities (MCS), five persons who had chronic
fatigue syndrome (CFS) and six controls. MCF and
CFS subjects reported more symptoms associated with
MTBE.
Neuropsychological symptoms among tanker drivers
with exposure to solvents were previously studied in
Finland.6 Exposure of road tanker drivers to organic
solvents was measured, and symptom questionnaires
were administered. The findings showed that drivers
had more acute symptoms which might be caused by
solvent exposure in comparison to the control group
who had no exposure to solvents. The differences
between the groups, however, were not statistically
significant.
The present study has two objectives: (1) assessment
of the occurrence of neuropsychological symptoms
and moods among tanker drivers during the previous
week and month and the associations of the symptoms
with exposure to gasoline and (2) comparison of the
occurrence of the symptoms and moods between
tanker drivers and milk delivery drivers;
CHEMICAL PROPERTIES OF GASOLINE
Gasoline is a liquid mixture. The composition varies
depending on the origin and quality of the raw oil and
the manufacturing process.7 Gasoline contains a number of easily volatile hydrocarbons. In analytical studies
150-200 hydrocarbon compounds in common gasoline
have been reported. A maximum of 30% are aromatic
hydrocarbons. About half of the compounds in gasoline
are aliphatic hydrocarbons such as paraffins. In Finl a n d gasoline c o n t a i n s a p p r o x i m a t e l y 10%
methyl-tert-buthyl ether (MTBE). MTBE acts as oxygenator to reduce carbon monoxide emissions and in
unleaded gasoline as octane enhancer.
In liquid gasoline the content of aromatic hydrocarbons is 25-30%; and in the vapour phase, around
2-4%. The content of MTBE remains the same in the
liquid as in the vapour phase.
THE EXPOSURE OF TANKER DRIVERS TO
GASOLINE
In the year 1994, 2.0 million m3 gasoline was sold in
Finland in 2,000 service stations. Gasoline is delivered
from stocks of oil firms to service stations with road
tankers, of which there are about 500 in Finland.
In Sweden the occupational exposure limit OEL (8h)
for gasoline is 220 mg/m3 and OEL (15min) 300
mg/m3.9 In Finland there is no specific occupational
exposure limit for gasoline.
The Oil Companies' European Organization for Environmental and Health Protection (CONCAWE) has
compiled the results for occupational hygiene measurements of road tanker drivers from European
countries in the 1980s.8 The mean exposure to total
hydrocarbons of gasoline during top loading («=142)
in those countries was 451 mg/m3 (range=6.4-3030),
and to benzene 6.1 mg/m3 (range=dl (detection
limit=60.5). The mean exposure of road tanker drivers
to total hydrocarbons during bottom loading («=59)
was 79 mg/m3 (range=8.2-234) and to benzene 1.4
(range=dl-55).
Hakkola and Saarinen (1996) have reported on the
exposure of the tanker drivers in the present study to
gasoline and some of its components.9 Exposure of
tanker drivers takes place during loading in the stocks
and during delivery in the service stations. The tanker
drivers fill self tanks. The exposure of road tanker
drivers to gasoline during loading depends on the
loading technique and climatical conditions.
In Table 1 is summarized the results of the exposures
of tanker drivers to gasoline and some of its components in the study.
MATERIAL
The group of tanker drivers in this study was selected
from all the drivers within three oil firms located in
six towns in various areas of Finland. The control
group of milk delivery drivers was selected from the
same six towns from two milk companies. The total
Table 1. Summary of the exposures of tanker drivers to gasoline and some its components
Bottom Loading
Number of samples
Sampling period (minutes)
mean
range
C3-C11 hydrocarbons (mg/m3)
mean
range
Benzene (mg/m3'
mean
range
MTBE (mg/m3)
mean
range
6
Top Loading
4
Delivery
Delivery
5
6
28
20
33
26
15-40
10-30
22-44
10-37
44
551
64
437
7-140
121-1,414
17-141
61-628
1.1
18
1.5
0.2-3.5
3-43
0.4-2.3
11.0
1.0-17.0
13.0
2.8-42.0
91
20-226
16.0
4.3-27.0
10-98
71
M. Hakkola et al.: Neuropsychological symptoms among tanker drivers exposed to gasoline
127
Table 2. Formation of the study groups
Tanker drivers
original sample
refused
not possible to take part in the study for other reason
not completed
final study groups
n
%
121
100
2
12
2
83
3
14
3
101
Table 3. The age distribution of subject groups
Age
(years)
<20
21-30
31-40
41-50
51-60
61-64
Tanker drivers
n
%
0
37
30
22
9
2
101
8.9
2.0
100
15
1
100
1
31
1.0
31.0
30.0
22.0
15.0
1.0
100
size of the sample was 269 drivers. The drivers were
asked by letter to participate in the interviews and
after that were contacted by phone by the researchers.
The final group of drivers who participated the study
was 201 (75% of the original sample). The percentage
of drivers who refused to participate in the interview
was 8%. The others had practical matters hindering
them from participating in the interviews. The final
group of subjects consisted of 101 male tanker drivers
and 100 male milk delivery drivers.
Table 2 shows the formation of the study groups
among tanker drivers and milk delivery drivers. Table
3 shows the distribution of age in the subject groups.
The mean age was 38.4 years, (tanker drivers=39.6
and milk delivery drivers=37.2). Among the drivers,
more were under 30 year olds and over 50 year olds
in the milk delivery group.
METHODS
The design of the study was cross-sectional. There
was no follow-up with either the drivers or the nonrespondents.
The drivers were interviewed twice: at home before
the work week, and at the end of the last work day of
the same work week at their place of work. The mean
duration of the first interview was 30 minutes and the
second interview 20 minutes. In this article the results
of the second interview are reported.
Items of the interviews
Questions related to work: Work history in the occu-
pation of driver, work hours, allocation of the shift
into various tasks, and driving distance. The second
interview inquired into specific exposure events for
gasoline during the work week.
All
148
19
100
13
269
22
25
17
39
4
100
3
7
68
201
Questions related to health.
Milk delivery drivers
n
%
0
20.8
31.7
36.6
21
32
Milk delivery drivers
n
%
100
8
14
3
75
Perceived health, chronic
diseases, use of medication, sick leaves. The second
interview addressed the health during the work week
and the use of medication during the work week.
Symptoms possibly related to exposure to gasoline.
The
symptoms related to the exposure of drivers to gasoline
were solicited using a screening method of neuropsychological symptoms developed by the Finnish
Institute of Occupational Health. The standard assessment method consists of neuropsychological tests and
questionnaires.10'11 The validity and reliability of the
method have been tested separately for questionnaires
and neuropsychological tests in many occupational
groups exposed to solvents at work. However, neuropsychological tests were not used in this study. Modified
POMS and symptom blank were used as questionnaires in the interview process.
There were two parts in the inquiring process: Part
1 contained questions on six symptom categories. The
scales were (numbers of items given in parentheses):
sleep disturbance (3), fatigue (5), memory disturbances (4), emotional distress (6), somatic complaints
(5), and sensory and motor symptoms (8). Part 1
concerned neuropsychological symptoms during the
last month and used three alternatives: 'hardly ever',
'now and then, or to some degree', and 'often, or to
a disturbing degree', scored respectively as 0,1 and 3.
Part 2 inquired about the occurrence of moods during the last week, representing a modification of the
Profile on Moods States (POMS) method. When the
original POMS was translated into Finnish, items
which did not have a corresponding mood-describing
adjective in Finnish were omitted (there are fewer
adjectives for the depressive mood in Finnish than in
English). A factor analysis was conducted in the study
of the Finnish Institute of Occupational Health with
the remaining 45 items (n=205), and new mood scales
were composed using the best items of each eight
factors obtained in the analysis. The scales were (numbers
of items given in parentheses): tension/anxiety (4),
fatigue (3), absent-mindedness (3), vigor (6), depression (7), hostility (7), listlessness (3), uncertainty (5).
Scales fatigue and listlessness were together. In this
paper, mood states obtained by modified POMS method
are conceptualized as neuropsychological symptoms.
There were five response alternatives ranging from
'never' to 'extremely', scored from 0-4. Table 4 shows
the scales and items of the modified POMS method
and response alternatives.
128
Occup. Med. Vol. 46, 1996
Table 4. Scales and items of the modified POMS. Alternatives
for answers: 0=Not at all; 1=A little; 2=Moderately; 3=Quite a
bit; 4=Extremely
RESULTS
Background
Scale
Tension-anxiety
Items
Anxious
Restless
Tense
Nervous
Fatigue
Worn out
Fatigued
Exhausted
Absent-mindedness
Unable to concentrate
Forgetful
Uncertain about things
Vigor
Active
Energetic
Cheerful
Enthusiastic
Vigorous
Alert
Depression
Unhappy
Sad
Depressed
Hopeless
Blue
Lonely
Distressed
Hostility
Angry
Peeved
Annoyed
Resentful
Touchy
Ill-humoured
Furious
Listlessness
Listless
Weary
Bushed
Uncertainty
The mean duration of work as drivers was 15.7 years
(range=2.0-40.0) among tanker drivers and 14.0 years
(range= 1.0-37.0) among milk delivery drivers.
Twenty-seven (27%) of the milk delivery drivers and
16 (15.8%) of the tanker drivers had worked under 5
years. The distribution of duration of work history as
a driver was reasonably comparable between the two
groups.
The mean duration of the work day was 9.9 hours
(range=8.0-12.0) among tanker drivers and 9.2 hours
(range=6.0-14.0) among milk delivery drivers. More
milk delivery drivers had prolonged work days than
tanker drivers. Sixteen (16%) milk delivery drivers and
two (2%) tanker drivers worked over 11 hours daily.
Ninety-four (93.1%) tanker drivers had regular twoshift work. Sixty-six (66.0%) milk delivery workers
were regular day-workers and 25 had their main working hours outside 6.00 am-6.00 pm.
Confused
Muddled
Uncertain about things
Puzzled
Helpless
Standard statistical methods were used. One-way
covariance analysis was used to age-adjust symptom
scales and to test the differences in scales between
subject groups. Dichotomized age classification under
and over 40 years was used. The Mann-Whitney test
was used to test the significance of the unadjusted
association between other factors than exposure of
gasoline and symptoms. One-way covariance analysis
was also used to test age-adjusted differences between
subject groups in symptoms affected by factors other
than exposure to gasoline.
Health and diseases
The bulk of drivers perceived their health as good or
very good; 35.6% of the tanker drivers and 30% of
the milk delivery drivers perceived their health as
average, bad or very bad. Chronic diseases were
reported by 25.8% of the tanker drivers and by 30.0%
of the milk delivery drivers. The occurrence of chronic
diseases and perceived health was not statistically
significantly different between the two groups.
Seven tanker drivers (6.9%) and 11 milk delivery
drivers (11.0%) took regular or almost regular medication. Psychoactive drugs were not used by the drivers.
Occurrence of symptoms
Tanker drivers scored higher than milk delivery drivers
on the fatigue symptom scale (3.6/3.2), and sensory
and motor symptom scale (1.2/1.0), adjusted for age.
Conversely milk delivery drivers had more somatic
complaints on average (1.0/0.8). However the differences
between the groups were not statistically significant
(Table 5).
In the modified POMS scales, tanker drivers scored
higher in the age-adjusted scales of tension/anxiety
(6.1/5.9), fatigue (6.5/6.4), absent-mindedness (4.5/4.2),
and listlessness (4.5/4.3). Milk delivery drivers were
higher in the scales for vigor (23.2/22.4), and uncertainty (6.8/6.7). The differences in these results between
subject groups were not statistically significant (Table 6).
The age of the drivers, chronic diseases and the
perceived health of the drivers were connected to the
occurrence of symptoms and modified POMS scales
among drivers. Duration of work as a driver, shift
schedule and length of the work week had no statistical
connection with symptoms and the modified POMS
scales.
M. Hakkola et al.: NeuropsychologicaJ symptoms among tanker drivers exposed to gasoline
129
Table 5. The mean age-adjusted symptom scores in subject groups
Scale
Tanker drivers (n=101)
Milk delivery drivers (n=100)
Scale value mean
Scale value mean
1.0
3.6
1.0
3.2
2.0
2.0
0.8
1.8
1.2
1.0
ns
ns
ns
ns
1.8
1.0
ns
ns
Tanker drivers (n=101)
Milk delivery drivers (n=100)
P*
Scale value mean
Scale value mean
6.1
6.5
4.5
22.4
9.0
10.0
4.5
6.7
5.9
6.4
4.2
23.2
9.0
Sleep disturbances
Fatigue
Memory disturbances
Somatic complaints
Emotional distress
Sensory and motor symptoms
P'
NS: not significant (p>0.05)
Table 6. Mean age adjusted modified POMS scores in subject groups
Scale
Tension/anxiety
Fatigue
Absent-mindedness
Vigor
Depression
Hostility
Listlessness
Uncertainty
10.0
4.3
6.8
ns
ns
ns
ns
ns
ns
ns
ns
* NS: not significant (p>0.05)
The groups were comparable in mean alcohol consumption. Alcohol consumption was not associated
with symptoms in the study.
between the two groups. The mean duration of work
days was similar.
Results
DISCUSSION AND CONCLUSION
Material
The target group for the study consisted of 101 road
tanker drivers and the control group consisted of 100
milk delivery drivers. The group of tanker drivers in
this study was selected from all the drivers within the
three largest oil firms employing drivers located in six
towns in various areas of Finland. The control group
was selected from the same towns from two large milk
companies. The criteria for selection was employment
in those firms in those towns. The sample was selected
from various areas of Finland and is representative of
Finnish tanker drivers and milk delivery drivers.
The percentage of drivers who refused to participate
in the interview was among tanker drivers 2% and
among milk delivery drivers 13%. The tanker drivers
may have been more motivated than the milk delivery
drivers to participate because the study was concerned
with work conditions of tanker drivers.
The comparability of subject groups according to
the questions of the study was satisfactory. In population characteristics and extraneous factors affecting
end points there were no statistically significant differences between the groups. The distribution of
duration of work as a driver was reasonably comparable
Comparisons of the results of the present study with
earlier studies are difficult because there are few studies
made with the similar methods and because of differences in exposures. In former studies on continuous
solvent exposure inside factories, 10 ' 11 symptoms
(fatigue, memory disturbances and sleep disturbances)
and POMS blank answers (fatigue, absent-mindedness, hostility and vigor) were connected with high
exposure to solvents. Exposure-response gradients were
observed. These scales also related to the psychological
stressors at work.
In our study there were no statistically significant
differences in the occurrence of symptoms between
tanker drivers and milk delivery drivers.
Mohr, et al. have studied self-reported symptoms
among workers exposed to MTBE. 4 Their findings did
not show more symptoms among workers with high
MTBE concentration environments than those workers with low MTBE concentration environments. The
MTBE volume in gasoline that workers handled was
up to 15%. In Finland MTBE content in gasoline is
circa 10%. In our study tanker drivers who were exposed to gasoline and MTBE did not have more
statistically significant symptoms than milk delivery
drivers. The scale of symptoms were different in these
two studies. Moreover, in the Mohr, et al. study the
exposure of workers to MTBE was examined more
130 Occup. Med. Vol. 46, 1996
accurately and the level of exposure was higher than
in our study.
Fiedler, et al. have studied the response of sensitive
groups to MTBE. 5 Those persons who had multiple
chemical sensitivities and so-called chronic fatigue syndrome had more symptoms when exposed to MTBE
compared to a healthy control group. In our study
drivers in both the exposed and unexposed groups
who had chronic diseases and poor health had more
symptoms.
In our study alcohol consumption had no statistical
connection to the results of the symptoms queried.
Moreover, the subject groups did not differ statistically
in alcohol consumption. Previous studies have shown
alcohol consumption to affect the occurrence of symptoms among workers exposed to solvents at work.10'11
The mean alcohol drinking frequency during one week
was 1.4 among drivers, and was 1.6 among men in
Finland between the age 20-69. 13 Abstinence rates
(those who according to self-reports had not consumed
alcohol in the 12 months preceding the interview)
were 3% among drivers and 10% as a whole among
men in Finland.
Although we found tanker drivers to have more
sensory and motor symptoms in comparison to milk
delivery drivers, the difference was not statistically
significant. Knave and Knave, on the other hand, conducted neurophysiological studies among workers
exposed to jet fuel. Their findings revealed a lowered
nerve conducting velocity among those workers.2'3
Moreover, a previous study has reported typical
symptoms characteristic of workers exposed to a high
level of solvents.10 In comparison, at the level of exposure to gasoline used in our study, we did not find
a statistically significant excess of symptoms which
are typical of workers exposed to solvents. The difference in these findings may be due to variable factors,
which were controlled for in our study: age, perceived
health and chronic diseases. These factors must be
taken into consideration in this kind of interview study.
ACKNOWLEDGEMENT
The study was financially supported by the Finnish
Work Environment Fund.
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