•
Introduction
Exposure
to chromium compounds in occupational settings includes workers involved
in the mining of chromite ore, the processing of the ore to chromium
chemicals, the production of alloys containing chromium and the use
of such compounds in processes such as metal plating, leather tanning,
the manufacture of pigments and in medical prostheses.
There is a wide range of compounds of chromium and they vary in physical
and chemical properties, hence the basis for their different uses
and applications. In nature, chromium exists primarily in the stable
trivalent state [1]
. Hexavalent chromium in the environment results almost exclusively
from human activities. Under normal environmental conditions, hexavalent
chromium tends to be reduced to the trivalent form relatively quickly.
The processing of chromite ore can produce both trivalent and hexavalent
chromium compounds. Chromium in alloys such as stainless steel is
in its metallic form (valency 0). The relevance of the different valency
states of chromium to possible health effects lies in the difference
in biological and toxicological properties between trivalent and hexavalent
chromium and its metallic form. There are also differences related
to solubility and bio-availability of the various compounds.
Trivalent chromium is an essential trace metal in humans - a property
similar to other metals such as cobalt, copper, magnesium and iron
[1,2]
. The essentiality is in regards to enzymes involved in metabolic
processes. Trivalent chromium has an essential role in carbohydrate
metabolism; it is necessary for insulin function. In comparison, hexavalent
chromium compounds are toxic and have been linked to an increased
risk of lung tumours, both in animal studies and epidemiological studies
of exposed workers [3,4]
. Metallic chromium is relatively inert and has no evidence indicating
it to be carcinogenic [5]
.
In the workplace, preventive steps such as engineering design containment,
improved systems of work and hygiene measures have minimised occupational
exposure to hexavalent chromium compounds leading to a reduction in
the experience of health effects.
To measure and assist in the control of exposure at the workplace,
ambient air monitoring is often performed and the exposure levels
compared to national occupational exposure standards.
However, ambient air monitoring does not take into account all routes
of exposure e.g. ingestion and through the skin and mucosa.
Biological monitoring is therefore an added option for monitoring
total exposure by analysis of the amounts of chromium in biological
samples such as blood and urine [6]
. In addition, examination of exposed individuals on a periodic basis
through health surveillance allows an opportunity for early detection
of health effects, which can then lead to appropriate preventive intervention
to reverse, reduce or minimise such effects [7]
.
•
Biological Monitoring for Chromium Chemicals Exposure
Biological Samples
The requirements for an effective biological monitoring program [6]
include:
a)
Relevant exposure leading to systemic absorption.
b) The existence of valid methods for sample collection and analysis.
c) The availability of reference values for interpreting the results
obtained.
d) An understanding of background variation and dietary influences.
For
occupational exposure to chromium compounds, blood and urine samples
have been used for biological monitoring. Venous blood samples can
be collected and analysed for chromium in whole blood, in plasma,
the red cell fraction and in white blood cells [8,9]
. Urine samples will indicate chromium derived from both trivalent
and hexavalent sources. Some hexavalent chromium is reduced to trivalent
chromium either by alveolar macrophages following inhalation or in
the gastrointestinal tract after ingestion. Any residual hexavalent
chromium absorbed is transported by blood where extra-cellular reduction
to the trivalent form continues, or where it enters red blood cells
before it is converted to trivalent chromium. Trivalent chromium is
rapidly cleared from the blood and excreted in the urine. It is also
possible to analyse hair samples for chromium, but it is not practical
to use this biological medium for biological monitoring in occupational
health.
Findings from studies at a UK chromite ore processing plant
The UK Health & Safety Executive is currently in the process of considering
a biological monitoring guidance value for exposure to chromium compounds.
The feasibility of biological monitoring at the only UK chromite ore
processing plant was explored with the regulatory authority [10]
. The following steps and procedures formed important practical considerations
for implementing a biological monitoring programme for exposure to
chromium chemicals:
a)
Decision on what biological samples to collect. The biological
samples collected were venous blood for determination of chromium
in whole blood, plasma, erythrocytes, and leucocytes, and urine
for total chromium.
b)
Liaison with the laboratory. The laboratory used was the
government health and safety laboratory that was located away from
the plant site. Discussions were conducted with the laboratory on
the amount and timing of sample collection, equipment to use for
collecting the biological samples, precautions necessary to prevent
contamination and procedures for ensuring secure storage and delivery
of samples to the laboratory.
c)
Communication with the company and the workforce. Communications
with the workers and employer dealt with explanations on the rationale,
basis and procedures for biological monitoring, securing consent
for participation and arranging a time-table for sample collection.
Full information and instructions were provided to the trained staff
from the site medical centre on the exercise.
d)
Development of an individual consent form and health questionnaire
which was provided to each participant. The questionnaire collected
items of information that would assist in interpretation of the
findings from the monitoring. They included items such as age, sex,
work details and diet.
e)
Participants presented themselves before the beginning and at
the end of their work-shift. The consent form, interviewer-administered
questionnaire and an explanation were provided before sample collection.
Precautions for preventing sample contamination were stressed.
f)
Collection and labelling of all samples which were kept secure
before despatch to the laboratory for analysis. The samples
were analysed for chromium content by atomic absorption spectrometry.
Urine samples were also analysed for creatinine content so that
the results could be expressed as nmol/mmol creatinine or as mg/g
creatinine. This use of creatinine correction adjusts for urine
concentration and dilution.
g)
Explanation of the findings to the workforce. When the results
were available, appointments were made to provide the results and
interpretation and any further explanations to each participant
on an individual basis. A presentation of the group results only,
with removal of specific identifiers, was also made to the management.
The main
findings from the most recent pilot exercise were:
a)
No significant difference in red blood cell or white blood cell
chromium between non-production workers, those involved mainly in
trivalent chromium processes and those in hexavalent chromium sections
of the plant.
b) Pre-shift plasma chromium levels higher in both groups of process
workers compared to the non-production staff.
c) No significant change in post-shift plasma chromium levels compared
to the pre-shift results.
d) Pre-shift urinary chromium levels higher in both groups of process
workers compared to the non-production staff.
e) Post-shift urinary chromium levels higher in the hexavalent chromium
group, compared to the other two groups and also higher when compared
to the pre-shift levels.
Feedback
obtained from the participants indicated that the workforce preferred
collection of urine samples to blood samples. Staff of the medical
centre also preferred urine sample collection to blood samples. Factors
against the collection of blood included the necessity for use and
subsequent disposal of hypodermic needles, the risk of contact with
blood and the precautions needed to prevent sample contamination.
No major concerns were expressed about the periodicity of biological
monitoring if it was implemented on no more frequently than an annual
basis. There were no major logistical problems experienced by the
management, workforce or medical centre and laboratory staff as part
of the biological monitoring process.
•
Health Surveillance for Chromium Chemicals Exposure
Health
Surveillance Procedures
The
principle behind health surveillance is to have in place a periodic
health assessment process that enables early detection of ill-health
effects. The detection of such effects should then lead to a review
of the systems of work and relevant preventive measures. The health
effects that are important to recognise in chromium exposure are primarily
those due to hexavalent chromium compounds. These include acute effects
such as nasal septal ulceration, irritation and sensitisation of the
skin and respiratory tract and chronic effects - the most important
of which is lung cancer. The clinical procedures that can be used
to detect these health effects include:
a)
Review of symptoms
b) Clinical Examination
c) Lung function tests
d) Chest X-rays
In the
UK, workers exposed to chromium compounds in defined work activities
are required by health and safety legislation to undergo periodic
health assessment. The relevant work processes include chromium plating
and the production of chromium chemicals from chromite ore. The system
currently used at the UK chromite ore processing plant provides for
pre-employment assessment, monthly symptom review, inspection of the
hands and nasal septum by an occupational health practitioner and
an annual health status review.
a)
Symptom review. Relevant symptoms are experience of cough,
chest tightness, wheeze, haemoptysis and unexplained weight loss.
These symptoms may indicate possible asthma and/or lung cancer.
Where symptoms and/or signs of health effects are experienced, a
system of referral to an occupational physician and specialist in
respiratory medicine or oto-rhino-laryngology is available. A referral
mechanism for obtaining further expert clinical advice through the
local and national hospitals has also been established. The monthly
visit to the medical centre is also used to reinforce the importance
of workplace and general preventive measures to the individuals
e.g. compliance with hygiene measures, use of appropriate personal
protective equipment, and cessation of cigarette smoking (as a risk
factor for lung malignancy and respiratory ill-health). Medical
centre staff also emphasize that individuals may consult them for
any concerns regarding workplace exposure or their health, with
no necessity to wait for a next scheduled appointment.
b)
Examination of the hands and nose. The aim of nasal inspection
is to identify the presence of nasal irritation before it proceeds
to ulceration and potentially to nasal septal perforation. Skin
ulceration can also occur from prolonged contact between chromates
and usually the exposed skin of the hands. With the improvements
in controlling exposure to chromates at the UK plant, there has
been a decline in the number of cases of ulceration and perforation
of the nasal septum and of ulceration of the hands or forearms.
Such cases are to be notified to the UK enforcement agency (the
Health and Safety Executive). Since 1990, there has only been one
case at this plant in 1993 of a laboratory worker with skin ulceration
that resulted from direct prolonged contact with chromates. (The
size of the workforce is around 200 workers in total).
c) Lung function tests. Use of spirometry to determine FEV1/FVC
ratio (Forced Expiratory Volume in 1 second/Forced Vital Capacity)
is done as a routine part of surveillance of respiratory health
or where there is a clinical suspicion of asthma. Individuals can
also be followed up with serial peak flow rate recordings. Spirometry
has not been shown to be of use for detection of lung cancer.
d) Radiology. The evidence for the efficacy of periodic chest
X-rays as a means of early detection of lung cancer from non-occupational
and occupational sources suggests that they are of limited value.
In addition to the disadvantage of giving workers a regular dose
of radiation, there is no significant gain in age of death for those
undergoing regular X-rays compared to those without. An analysis
of the experience of regular chest X-rays for chromate-exposed workers
at this UK plant has demonstrated no significant improvement in
lung cancer 5-year survival rates [11]
. A recent review of the use of radiology for statutory medical
examinations of workers exposed to respiratory carcinogens proposed
that the statutory requirement for regular chest X-rays for screening
be discontinued and only considered when there is a clinical indication
for its use [12]
. New imaging techniques such as spiral CT (computerised tomography)
scans may hold some promise for health surveillance in the future.
•
Discussion
The
experience at the only UK plant processing chromite ore indicates
that it is feasible to conduct air/biological monitoring and health
surveillance for exposure to chromium compounds in the various processes
at the plant. This is supported by the views and experience of occupational
health professionals in other countries [13]
.
Discussions are currently taking place with the UK regulatory authorities
to decide on a need and standard for biological monitoring for chromium
exposure. Urinary chromium would appear to be the most practical biological
monitoring requirement to implement. Apre- to post-shift rise of urinary
chromium levels has advantage over a single end-of-shift urinary sample.
The American Conference of Governmental Industrial Hygienists (ACGIH)
already has a Biological Exposure Index for urinary chromium of 10
µg/g creatinine as an increase during the shift and 30 µg/g
creatinine at the end of shift at the end of the workweek [14]
. There is a proposal to change the indices to 10 µg/L and 25
µg/L respectively. The UK Health and Safety Executive (HSE)
could well use this as a guide for introducing a Biological Monitoring
Guidance Value (BMGV) for urinary chromium. As of 2002, there are
only thirteen chemicals for which BMGVs have been provided by the
HSE [15]
.
Health surveillance is useful for detecting and preventing acute health
effects but its effectiveness for chronic health effects such as lung
cancer is doubtful.
The procedure should support an effective control of workplace exposure.
Confirmation of the absence of an excess of lung cancer can be done
through epidemiological analysis.
Since the early observation of an excess of lung cancer experience
at this UK plant [16]
, progressive improvements in workplace hygiene and preventive measures
have led to a reduction of worker exposure to chromates, with a subsequent
demonstration of no-excess lung cancer experience [17]
.
An epidemiological update of the mortality experience of the workforce
is planned. Epidemiology is an effective additional tool to complement
biological monitoring and health surveillance in confirming an absence
of significant exposure and health effects.
•
Conclusion
The
use of biological monitoring to complement air monitoring for assessing
exposure and health surveillance and detecting early health effects
is feasible for workers involved in the processing of chromite ore
to chromium chemicals.
Before instituting health surveillance or biological monitoring in
any industry, factors such as the extent and likelihood of exposure,
an understanding of the limitations of these procedures, the availability
of competent staff, instrumentation, equipment and laboratories and
a means of interpreting the findings are absolutely essential.
•
References
