•
Summary
•
Objectives - To determine whether occupational exposure to chromite,
trivalent chromium or hexavalent chromium causes respiratory diseases,
an excess of respiratory symptoms, a decrease in pulmonary function
or signs of pneumoconiosis among workers in an integrated chain of
stainless steel production.
•
Methods
- This cross sectional study was carried out in 1993 and the inclusion
criterion was a minimum of eight years of employment in the same production
department. A self-administered questionnaire was collected and spirometry,
measurement of diffusing capacity, chest radiography and laboratory
tests were carried out by a mobile research unit.
•
Results
- There were 221 workers in the exposure groups and 95 in the control
group. The average duration of employment was 18 years. No significant
differences in the odds ratios (ORs) of the symptoms were found between
the exposure and the control groups. In a logistic regression analysis,
age and smoking significantly explained the occurrence of most of
the respiratory symptoms. The smokers in the chromite group had significantly
lower forced vital capacity (FVC), forced expiratory volume in one
second (FEV1) and diffusing capacity than the corresponding values
of the control group. The analysis of variance between study groups,
smoking and exposure time, without modelling for interactions, showed
that the chromite group had lower values for FVC, FEV1 and diffusing
capacity than the other groups. The occurrence of small opacities
was more frequent on the chest radiographs of the workers in the chromite
group.
•
Conclusions - An average exposure time of 18 years in ferrochromium
and stainless steel production and exposure to dusts containing low
concentrations of hexavalent or trivalent chromium do not lead to
any respiratory changes detectable by lung function tests or radiography
nor to any increase in symptoms of respiratory diseases. The lung
function values were lower and the occurrence of radiological findings
was more frequent among the workers from the chromite mine than among
the controls. The difference was partly caused by differences in age
and smoking habits, but evidently also partly by higher exposures
more than two decades ago or by the fibrous components of the dust.
This
report first appeared in Occupational and Environmental Medicine
1996;53:741-47. Reprinted by kind permission.
Chromate
dust and fumes of chromium trioxide have been reported to cause asthma.(1)
Chromates, among other exposure agents in chromium plating, welding,
and ferrochromium production, have been connected with cases of occupational
asthma(2) or bronchitis.
Mining dust in an underground chromite ore mine has also been reported
to cause chronic bronchitis.(3)
Obstructive
effects on lung function have also been found among chromium workers.(4,6)
In one study reduced forced vital capacity (FVC) and an increased
prevalence of obstructive lung diseases were found among electrofurnace
workers in a ferrochromium plant.(7)
The author suggested that the effects were due to high levels of total
dust, especially amorphous silica dust.
Nodular
pneumoconiosis is another finding among workers in chromate production.(8,9)
However, it has not been confirmed in some studies.(7,10)
In a study on respiratory symptoms among 60 ferrochromium workers
in Norway pneumoconiosis was diagnosed from the radiographical examinations.(7)
The integrated
production chain of Outokumpu Steel Oy is unique; the mine and all
the stainless steel manufacturing plants are in the same region. The
figure shows a flow chart of the process. Unlike the rest of the ferrochromium
furnaces in the world, the Outokumpu process is a fully closed system.
In an
animal experiment(11)
chromite particles from Outokumpu's chromium mine were considered
to be fibrogenetically inert.
The purpose
of the present study was to determine whether long term occupational
exposure to low levels of chromite, trivalent chromium (Cr+3), or
hexavalent chromium
(Cr+6) causes respiratory diseases, an excess of respiratory symptoms,
a decrease in pulmonary function,
or signs of pneumoconiosis among workers in an integrated chain of
stainless steel production, and to investigate whether Cr+3 and Cr+6
compounds (known to differ in their toxicity) also differ from each
other in this respect.
•
Subjects and methods
•
Subjects
At the
time of the study, from March to May 1993, there were 892 workers
in the production departments. The subjects were divided into four
groups according to exposure to different chromium compounds: exposure
to Cr+6, Cr+3, chromite, and a control group. In this study the inclusion
criterion was a minimum of eight years of employment in the same department.
Altogether 222 workers met the criteria of exposure and duration of
employment and were placed in one of the three exposure groups. One
of the workers did not agree to participate in the study. The control
group consisted of workers from the cold rolling mill (the Sendzimir
rolling mill, the skin pass mill, and the splitting
and cutting line), because their level of exposure
to chromium or dust in general was extremely low - or non-existent.
Out of the 132 potential controls, 95 agreed to participate. The lower
response rate among controls was mainly due to the relative inconvenience
to the workers in matching workshifts with study appointments. The
total number of subjects was therefore 316 and all of them were men
with similar ethnic and socioeconomic backgrounds.
The company
also provided a list of names of the workers who had been working
for at least eight years in any of the production departments under
study and then resigned. There were 53 such workers; 17 of them had
been exposed to Cr+6, four to Cr+3, and 19 to chromite; 13 of them
had been working in the cold rolling mill where the control group
worked. A questionnaire was sent to all of these former workers, but
no clinical examinations were made.
•
Exposure
An exposure
study was carried out through the whole production chain in 1987.(12)
At the chromite mine in Kemi (chromite group) the average dust concentration
was 1 mg/m3. The median personal exposure to chromium was 22 µg/m3;
Cr+6 was not detected in any of the samples.
In the
furnace department of the ferrochromium plant (Cr+6 group) the average
dust exposure was 1.5 mg/m3. The dust contained an average of 5% -
10% chromium. The proportion of Cr+6 of the total chromium was 0.1%
- 0.3%. The highest concentrations were detected during tapping in
the vicinity of the tap hole, where the proportion of Cr+6 was 10-fold
(1% - 3%) the level in other areas. In the sintering and
crushing departments (Cr+3 group) the average dust exposure was 2.4
mg/m3.
In the
steel smelting shop (Cr+6 group), the average exposure to total dust
was 1.8 mg/m3. The dust contained 2% - 4% chromium. The median Cr+6
concentration was 0.5 µg/m3. The highest values among personal
samples were detected in the handling of molten metal by the arc furnace
(6.6 µg/m3). Although judged from the stationary samples (in
which the amount of air collected
was large), Cr+6 seemed to be present at low concentrations throughout
the steel smelting shop; it exceeded the detection limit of 0.5 µg/m3
in only some of the personal samples.
The total
dust content was low throughout the cold rolling
mill (control group); it averaged 0.3 - 0.5 mg/m3. In general, the
content of chromium in the air in the cold rolling mill was lower
than the detection limit of the measurement method.
•
Methods
A questionnaire
was sent to the participants one week before the clinical examinations.
It was based on the standardised questionnaire of a study made in
the wool textile industry by the Edinburgh Study Group(13)
and the definitions from the Medical Research Council (MRC) questionnaire(14),
and it asked for information on personal characteristics, occupational
history, respiratory symptoms, smoking habits, medication, and personal
and family histories of allergic and pulmonary diseases. Some questions
considered, among other things, the following items: cough, phlegm,
shortness of breath, and wheeze. Symptoms of rhinitis and eye irritation
were also included.
Cough,
lasting more than three months and improving after a holiday of more
than one week, was considered to be work related. Similarly, dyspnoea
occurring at least twice a month and caused or worsened by impurities
in he
work environment or during a workshift, but becoming
better after a week's holiday, was regarded as work related. Questions
about back or stomach pain and general health status were used as
control questions
because they were not considered to be associated
with the current occupational exposure.
Spirometry,
measurement of diffusing capacity, a chest X-ray film examination,
and laboratory tests were carried out by a mobile research unit with
two experienced
laboratory technicians.
Spirometry
was performed on each subject with a computerised flow volume spirometer
ME 101 (Medikro Oy, Kuopio, Finland). The spirometer was calibrated
each day with a 5 litre syringe. At least three satisfactory acceptable
forced maximal expirations were performed according to the standards
of the American Thoracic Society,(15)
and all volumes were corrected to body temperature, pressure, and
saturation (BTPS). Each subject was seated wearing a nose clip. From
the maximum expiratory flow volume curves the highest FVC, forced
expiratory volume in one second (FEV1), and flow rates at 50% and
25% of the vital capacity (MEF50, MEF25) and their mean flow (MMEF)
were read.
All of
the values were also expressed as percentages of predicted values
in Finland.(16) Two
trained laboratory technicians examined an equal number of people.
The technicians' performance of spirometry was compared before the
study. The mean difference in the FVC of the people tested was minimal
(2.1%).
Diffusing
capacity of the lungs for carbon monoxide (Tlco) was measured with
the Morgan transfer test and the single breath method. Alveolar volume
(VA) is the total lung capacity at the time when the Tlco is measured.
The specific diffusing capacity is Tlco/VA. The same laboratory technician
performed at least two successful consecutive measurements for each
person; the mean value of the two nearest test results was chosen.(17)
These values were adjusted to the real time haemoglobin measurement.(18)
The results were also expressed as percentages of predicted values
in Finland.(16)
In the
radiographic examination full size 35 x 43 cm X-ray films were used.
The radiographs were classified according to the modified classification
system of International Labour Organisation (ILO).(19)
Two radiologists,
both of whom were experienced with the ILO system, classified the
radiographs individually without knowing the names or exposure data
of the subjects.
If the classifications of the radiologists differed, the higher classification
was recorded as the result. One of the radiologists was a certified
NIOSH B reader. The radiographs of the control group were mixed
with those of the exposed groups.
The urinary
concentration of chromium was measured from the specimens of 44 workers
from the steel smelting shop to ascertain the exposure level compared
with the level in an earlier study in the same production chain.(12)
Every other worker in the steel smelting shop was selected from the
alphabetical payroll list. The specimens were collected in the afternoon
after the workshift.
•
Statistical methods
Basic
statistics were used to describe the data. The frequency
tables were analysed with 2 statistics. The multivariate analysis
was based on logistic regression analysis, where we have included
possible confounders as predictors and their effect was adjusted,
when needed. The effect of risk factors in the model were shown with
adjusted odds ratios. The natural confounders in our study were age,
exposure time, smoking, earlier lung disorders, and atopic diseases.
When the relation between pulmonary functions and exposure were studied,
we used Student's t test and analysis on covariate. Here the possible
effect of confounders were taken into account using covariates. The
possible confounders were age, exposure time, height, weight, and
smoking. As a covariate, smoking was taken into the model as pack-years,
but we also included it in the model as a factor, where it was classified.
The statistical software used was SAS (SAS Institute, USA) and Egret
(Egret, USA).
•
Results
•
Subjects
The participation
of exposed workers in this study was high, almost 100%, and for the
controls the corresponding value was 72%.
The study
groups were similar in height and weight (table
1). There was a five year difference between the mean age
of the youngest (control) and the oldest (Cr+3) groups. More than
half of the workers in the Cr+3 and chromite groups were between the
ages of 45 and 65 years. The chromite group contained more current
smokers and fewer non-smokers than the other groups. Also, the chromite
group smoked more and had smoked longer than the other groups.
Only
three men had been previously exposed to chromium compounds in the
metal industry. The groups were similar for former agricultural work
and exposures to silica dust, welding fumes, asbestos, solvents, and
textile dusts (data not shown). There were no differences between
the number of reported earlier allergic or pulmonary diseases diagnosed
by a physician. Allergic rhinitis and bronchial asthma were infrequent;
no asthma was reported in the chromite or control group. The groups
were similar with regard to medication for hypertension and cardiovascular
diseases.
•
Symptoms
The prevalence
of most of the respiratory and other symptoms did not differ significantly
in the comparison between the groups (table
2). The production of phlegm was more frequent in the Cr+3
and chromite groups than in the other groups. Work related cough or
dyspnoea was significantly more frequent in the Cr+6 (P = 0.041) and
the Cr+3 (P = 0.033) groups than in the control group.
No significant
differences between the exposure groups and the controls were found
in the odds ratio (OR) of the symptoms (table
3). In the logistic regression analysis age and smoking significantly
explained the occurrence of most of the respiratory symptoms. Earlier
allergic diseases were associated with the occurrence of shortness
of breath (dyspnoea).
•
Lung function tests
In general
the smokers showed lower lung function results. The smokers in the
chromite group had significantly lower FVC, FEV1, and diffusing capacity
values than the smokers in the control group. The results of the lung
function tests (as the percentage of the predicted values) of the
Cr+6 and Cr+3 groups did not differ from the corresponding results
of the controls, except for the FVC of the smokers in the Cr+3 group,
for whom it was lower than for the smokers in the control group (table
4).
In the
group comparison of the adjusted lung function results, modelling
for interactions between study group, smoking and exposure time did
not show a significant difference for any of the lung function variables.
The same
analysis of variance without interactions showed
that the chromite group had lower FVC, FEV1 and diffusing capacity
values than the other groups, including the control group. The difference
in these variables remained when smoking (pack-years) was included
as a covariate in the model. The inclusion of smoking or pack-years
or both significantly decreased the values of the same variables.
The percentages
of the lung function test results that were below the predicted values(16)
were similar among the exposed groups, except for the chromite group,
for which all the diffusing capacity variables and the FVC, FEV1,
and MEF50 were significantly decreased (table 5).
•
Radiographs
Table
6 shows the radiological findings. The number of positive
findings increased with age in the Cr+6, Cr+3, and control groups.
Radiographic
parenchymal abnormalities were found more often, but not significantly
among the workers in the chromite group (table
7).
Changes
in the parietal and visceral pleura were also more frequent (not significantly)
among the workers exposed to chromite, who, however, had no bilateral
plaques (table
8).
•
Urinary
chromium
The mean
urinary concentration of chromium was 0.03
µmol/l for the 44 workers in the steel smelting shop (0.04 µmol/l
in the previous study in 1987(12).
The maximum
concentration was 0.08 µmol/l (0.34 µmol/l in 1987). The
results of these control measurements indicate that the level of chromium
exposure in 1993 was the same or slightly lower than in 1987.(12)
•
Former
workers
The questionnaire
was returned by 37 former workers (70%). They reported that they had
experienced the following
symptoms during the years they were working for the company: frequent
cough (16.2%), production of phlegm (16.2%), shortness of breath (13.5%),
rhinitis (35.1%), eye irritation (8.1%), dermatitis (18.9%), and various
symptoms (headache, vertigo, fatigue, etc. 16.2%).
None
of the former workers reported that a disease had been a reason for
leaving the company. At the time of the survey one person (2.7%) reported
having chronic bronchitis
and two people (5.4%) reported bronchial asthma. No other pulmonary
diseases, allergic rhinitis, or cancer were reported.
•
Discussion
The respiratory
effects of exposure to chromium compounds have been studied in various
industries, but none of the studies cover the entire production chain
of stainless steel. Because our objective was to study chronic effects,
the requirement for a minimum duration of employment had to be kept
high. Therefore the group sizes were small. This will decrease the
statistical power of the study, as can be seen in the wide 95% confidence
intervals (95% CIs) of the ORs for symptoms (table
9).
On the
other hand, the durations of exposure were long, the average ranging
from 16 to 20 years in the different exposure groups. Lowering the
durations of exposure would not have essentially increased the group
sizes.
Most
of the workers in this study had worked in the same production department
during their entire employment at Outokumpu Oy. Therefore the exposures
can be considered to the same for all the people in any given department,
occasional peaks being smoothed by time. Thus the scarcity of findings
in this study is not due to the possibility that the pathological
findings of a small, but highly exposed group had been diluted among
a larger and less exposed group.
All the
workers who had already left the company received a questionnaire,
and 70% of them returned it. Half of the 70% had smoked while employed
by the company. This group of former workers did not differ from the
other groups for respiratory symptoms. Chronic bronchitis among the
former workers was as rare as among the workers examined. Two former
workers
reported bronchial asthma, but none of the present workers did. None
of the former workers reported a disease as a reason for leaving the
company. Thus selection can be regarded as non-existent in this study.
The lung
function tests could not be carried out in a randomised
order for practical reasons. However, the lack of randomisation cannot
be considered to have had any notable effect on the results because
the tests were performed in a standardised manner.
Minor
differences in the occurrence of cough and shortness of breath were
seen when the Cr+6, Cr+3, and chromite groups were compared with the
control group. However, in the logistic regression analysis no significant
differences in the risk ratios were found between the groups.
The lung
function results of our study could not be compared with the results
from other studies because there are no such data available on stainless
steel production.
As in
this study epidemiological studies on respiratory functions very often
indicate that the results of lung function tests of former smokers
may surprisingly differ from the results of both non-smokers and current
smokers.
The reason for them having stopped smoking is often some effect on
the respiratory system. Therefore in this study ex-smokers were included
in the group of smokers in most of the analyses.
The results
of the lung function tests in the groups exposed
to Cr+6 or Cr+3 did not differ from those of the control group. On
the contrary, the results on respiratory volumes and diffusing capacity
in the chromite group were slightly worse than those of the control
group. This difference
was most evident for the smokers in this group, which had the largest
proportion of smokers.
The airborne
dust in the chromite mine has been shown to be one third chromite,
one third talc, and one third chlorite serpentine. The talc and serpentine
particles are primarily lamellar schists. Part of the chlorite serpentine
particles can be classified as fibres. During the late 1960s and early
1970s, when the process was different and the technology less advanced,
the dust concentrations in the ambient air at the mine were higher
than the current concentrations.
Limited
areas in the ore body of the north eastern part of the mine and its
waste rock contained crevices filled with chrysotile. During 1987-9
the mean concentration of fibres in personal samples was 0.14 fibres/cm3
for drillers and 0.11 fibres/cm3 for loaders at the open pit and 0.28
fibres/cm3 in the concentrating plant. According to scanning electron
microscopy 42.5% of these fibres were chrysotile asbestos, and the
rest comprised other particles
-
for example, vertical lamellar minerals.
The workers
from the chromite mine were slightly older than the controls, and
the proportion of smokers among them was greater. Therefore the lower
lung function values of these workers can be explained partly, but
not totally, by the differences in age and smoking habits between
the groups. It is also probable that there is a causal correlation
between our findings and both the higher dust exposures during the
first years of mine operation and the fibrous minerals in the waste
rock of the mine.
Radiological
parenchymal abnormalities and plaques classified according to ILO
recommendations were not common in our study. Small opacities were
more frequently
observed in the radiographs of the workers exposed to chromite than
in the radiographs of the other groups. This finding agrees with the
idea that early exposure
to fibrous materials is a factor contributing to the pulmonary effects.
An increased
risk of lung cancer has been found among workers in the production
of chromates and chromate pigments, as well as in chromium plating,
although no conclusive data are available on lung cancer in ferrochromium
production.(20, 21)
No cases of lung cancer were found in this study. However, risk of
cancer could not be excluded because the follow up time was short
and the exposed group young and small.
No cases
of chromium asthma were found in this study. It was recently reported
that mortality from non-malignant diseases of the respiratory system
was not increased (standardised mortality ratio (SMR 0.88)) among
production workers in stainless steel production.(21)
•
Conclusions
An average
exposure time of 18 years in modern ferrochromium
and stainless steel production and low exposure to dusts containing
Cr+6 or Cr+3 does not lead to any respiratory changes detectable by
lung function tests or radiography or to any increase in symptoms
of respiratory
diseases. The process chain under study is unique; however, the results
are also applicable to other production facilities where the exposures
to different chromium compounds are equally low.
The lung
function test results were lower and the occurrence of radiological
findings was more frequent among the workers from the chromium mine
than among the controls. The difference
was partly caused by differences in age and smoking habits, but evidently
also partly by higher exposures more than two decades ago, when the
mine operations were started,
and by the fibrous components of the dust.
Because
the follow up time was limited, we are planning a reinvestigation
after five years, although there are no personal needs or expected
benefits to individual workers.
The main reason is to collect new information and to confirm the present
results.
This
study was conducted with financial support from the Finnish Work
Environment Fund.
•
References
1
Meyers JB. Acute pulmonary complications following inhalations of
chromic acid mist. Arch Ind Hyg Occup Med 1950; 2:742-7.
2 Haines AT, Nieboer E. Chromium hypersensitivity.
In: Nriagu JO, Nieboer E, eds, Chromium in the natural and human
environments. New York: John Wiley, 1988: 497-532.
3 Ballal SG. Respiratory symptoms and occupational bronchitis
in chromite ore miners, Sudan. Journal of Tropical Medicine and Hygiene
1986; 5:223-8.
4 Reggiani A, Lotti M, De Rosa E, Saia
B. Impairments of respiratory functions in subjects exposed to chromium:
Note 1. Spirographic
changes. Lavoro Umano 1973; 25:23-7.
5 Bovet P, Lob M, Grandjean M. Spirometric alterations in workers
in the chromium electroplating industry. Int Arch Occup Environ Health
1977; 40:25-32.
6 Lindberg E, Hedenstierna G. Chrome
plating: symptoms, findings in the upper airways, and effects on lung
function.
Arch Environ Health 1983; 38:367-74.
7 Langård S. A survey of respiratory symptoms and lung
function in ferrochromium and ferrosilicon workers. Int Arch Occup
Environ Health 1980; 46:1-9.
8 Mancuso TF, Hueper WC. Occupational
cancer and other health hazards in a chromate plant: a medical appraisal.
I.
Lung cancer in chromate workers. Ind Med Surg 1951; 20:358-63.
9 Zoper A. Possible dangers to the respiratory tract from welding
fumes: methods of approach in an industrial health care context and
results. Schweissen Schneiden 1982; 34:77-81.
10 US Public Health Service. Health of
workers in chromate producing industry. Washington, DC; US Department
of Health,
Education, and Welfare, US Public Health Service 1953; 192:131.
11 Swensson A. Experimental research on the fibrogenetic effect
of chromite. Stockholm: Arbetarskyddsverket 1977. Arbete och Hälsa
1977; 2:1-14.
12 Huvinen M, Kiilunen M, Oksanen L,
Koponen M, Aitio A. Exposure to chromium and its evaluation by biological
monitoring
in the production of stainless steel. Occup Med Toxicol
1993; 3:205-16.
13 Love RG, Smith TA, Gurr D, Soutar CA, Scarisbrick DA, Seaton
A. Respiratory and allergic symptoms in wool textile workers. Br J
Ind Med 1988; 45:727-41.
14 Medical Research Council (MRC). Questionnaire
on respiratory symptoms, instructions to interviewers. London: MRC,
1986.
15 American Thoracic Society. Standardization of spirometry
(1987) update. Am Rev Respir Dis 1987; 136:1285-98.
16 Viljanen AA, Halttunen PK, Kreus K-E,
Viljanen BC. Reference values for spirometric, pulmonary diffusing
capacity and
body plethysmographic studies. Scand J Clin Lab Invest 1982; 42 (suppl
159): 1-50.
17 Make B, Miller A, Epler G, Gee JBL. Single breath diffusing
capacity in the industrial setting. Chest 1982; 82:351-6.
18 Cotes JE. Lung function. Assessment
and application in medicine. 3th ed. Oxford: Blackwell, 1975.
19 International Labour Office. International classification
of radiographs of pneumoconioses. Geneva: ILO, 1980.
20 International Agency for Research
on Cancer. IARC monographs on the evaluation of carcinogenic risks
to humans; Vol
49. Lyon: IARC, 1990.
21
Moulin JJ, Wild P, Mantout B, Fournier-Betz M, Mur JM, Smagghe G.
Mortality from lung cancer and cardio-vascular
diseases among stainless-steel producing workers. Cancer Causes Control.
1993;4:75-81.
*
M Huvinen - Outokumpu Oy, Espoo, Finland
J Uitti, P Roto - Tampere Regional Institute of Occupational
Health, Tampere, Finland
A Zitting, A Aitio - Institute of Occupational Health, Helsinki,
Finland
K Virkola - Helsinki University Hospital, Helsinki, Finland
P Kuikka - Outokumpu Polarit Oy, Tornio, Finland
P Laippala - School of Public Health, University of Tampere
and Tampere University Hospital, Tampere, Finland
