Introduction
One of us was asked to consider a diagnosis of occupational asthma for a man who had
worked for 20 years as a metal turner in a large, modern factory producing specialised
machine parts. He described a 2 year history of severe breathlessness that improved
when he was not at work. His spirometry was restrictive with a FEV1 of 1.35 L (40%
predicted) and FVC of 1.8 L (45% predicted), a ratio of 75%. Other lung function measurements
indicated gas trapping; his TLC was 5.01 L (79% predicted) and RV/TLC 170% predicted.
A high resolution CT scan of his lungs revealed a widespread ‘mosaic’ pattern of attenuation
indicative of small airflow obstruction. We made a diagnosis of occupational extrinsic
allergic bronchioloalveolitis and recommended that he change his work. After 12 months
working elsewhere in the same company, away from the machine shop, his dyspnoea was
greatly improved but had not disappeared; his FVC had increased to 2.41 L, his FEV1
to 1.45 L and his TLC to 5.36 L.
Four months later we were referred a man who was also a metal turner in the same factory.
For 2 years he had been a patient in a specialist interstitial lung disease clinic
with a diagnosis of chronic hypersensitivity pneumonitis. A marked lymphocytosis in
his bronchoalveolar lavage suggested ongoing exposure to an external cause. The nature
of this had not been established although the positive findings of an autoimmune screen
had led to conjecture of an ‘autoimmune’ aetiology, and of a high level of serum-specific
IgG antibodies to Aspergillus species, that exposure to ‘mould at home or work’ might
be relevant; an occupational history noted only that he worked for a machine parts
manufacturer. While continuing to work he had been treated with pulsed methylprednisolone,
cyclophosphamide, prednisolone, mycophenolate and N-acetyl cysteine with little evidence
of success. His referral was occasioned by a (new) physician noting that his symptoms
improved when he was not at work. On being informed that his illness was in all probability
caused by his occupation, he chose not to return to work. Six months later, without
any specific treatment, his lung function measurements had started to improve.
Following the first diagnosis, discussion with the factory's occupational health service
led to a systematic survey of 250 employees who worked in the same area. Through this
we established that another metal turner was a patient at a third hospital with a
diagnosis of hypersensitivity pneumonitis made 2 years previously; to his bemusement,
since he had never kept them, a probable attribution to ‘birds’ had been made. He
had been treated, intermittently, with high doses of prednisolone with no evidence
of lasting benefit. The survey of other employees and subsequent specialist investigation
established a further two cases of occupational bronchioloalveolitis with probable
onset in 2010–2011.
Discussion
These five men had a diagnosis and an occupation in common; all were metal machinists
in a single factory, a job that entails exposure to mists of metal working fluids
(MWFs). Inhalation of MWF is well recognised as a cause of extrinsic allergic bronchioloalveolitis
(in this context a more accurate term than hypersensitivity pneumonitis),1
2 but it appears, from this experience, that the association is not widely appreciated
by respiratory physicians.
Machining or ‘turning’ metal parts on a lathe is a skilled occupation used in the
manufacture of a very wide variety of components; this is often done using cutting
tools controlled by a computer and metalworking machinists in the UK and elsewhere
may describe themselves as ‘computer numerical control’ operators. Other terms include
computer numerical control grinder, turner, tool setter, cutter and machinist. Metal
shaping and grinding commonly involves the use of MWF (in the UK also known as ‘coolant’,
‘cutting fluid’ or ‘suds’) to lubricate the process, to control its temperature and
to carry away the waste metal (‘swarf’). The machines are generally enclosed and may
be exhaust-ventilated to reduce—but rarely eliminate—the escape of MWF mist into the
atmosphere. MWF is collected and recirculated, often with several machines sharing
a common ‘sump’ or reservoir. Systems of MWF management are required to replenish
and maintain its effectiveness and to monitor levels of microbial contamination. In
the factory above, new lathes had been installed in 2010; in contrast with those they
replaced, the new machines were capable of being operated continuously for 24 h and
used a far higher volume of MWF. These changes probably led to far higher concentrations
of MWF mist in the air of the shop floor.
Most modern MWFs are complex emulsions of water and a mineral, synthetic or semisynthetic
oil; they also contain a wide range of chemical additives designed to enhance their
performance and limit microbial growth. Allergic respiratory conditions due to water-containing
MWFs date back to the late 1980s, with challenge studies confirming cases of occupational
asthma caused by chemicals such as ethanolamine and pine oil deodorants.2 More recently,
a number of large outbreaks of respiratory disease have been reported in US and European
MWF-exposed workers, with cases of extrinsic allergic (bronchiolo)alveolitis (EAA),
occupational asthma, bronchitis and humidifier fever.2 Although difficult to prove,
the aetiology of the outbreaks is thought to be due to poor control of MWF mists,
linked with microbial contamination occurring during their recirculation and prolonged
use. Evidence to support this comes from the unpredictable nature of the outbreaks
and the demonstration of IgG antibodies to a range of bacteria, fungi and environmental
mycobacteria in the serum of exposed workers (with and without disease).3
A high index of suspicion is required in EAA due to MWF exposure as the symptoms are
often non-specific and may be progressive, rather than clearly work-related. In some
cases the presenting symptoms have been predominantly constitutional, with general
malaise and unexplained weight loss. Long delays in reaching the correct diagnosis
are not uncommon because symptoms are often attributed to asthma, COPD or to recurrent
chest infections; or, as here, the diagnosis has been otherwise explained. A HRCT
with inspiratory and expiratory views in a period when the patient is symptomatic
is probably the most useful diagnostic tool. This may show typical features of bronchioloalveolitis,
with one or more of ground glass opacities, small centrilobular nodules and lobular
areas of gas trapping4 although this last finding is not specific. In other cases
however, the HRCT may appear normal (particularly during periods of relatively low
exposure), or show a pattern of disease more suggestive of non-specific or usual interstitial
pneumonitis. As with any other cause of EAA, early recognition and the prevention
of further exposures offer the best outcome; as with all occupational diseases, every
effort should be made to maintain employment and relocate affected workers to a safe,
non-exposed work role.
An additional consideration is that the patient who has EAA from MWF may have a number
(sometimes several hundred) of colleagues with similar exposures.3 The diagnosis of
a single case should prompt the workplace to review their risk assessment and exposure
controls, and to survey the remaining workforce to identify other affected workers.
Investigating large outbreaks is logistically challenging, and where possible should
involve a multidisciplinary team including occupational lung disease specialists,
occupational health providers and occupational hygienists.5 In the outbreak above,
careful scrutiny of all exposed employees indicated that the problem was confined
to a small area of the shop floor; following extensive remedial works to control MWF
mists there have been no further cases.
Over the last decade, MWF exposure has become the most commonly reported cause of
occupational EAA in the UK, responsible for approximately half of all cases. We recommend
that all patients with suspected EAA (and other patterns of interstitial lung disease)
are routinely asked about MWF exposures, and that all potential cases are discussed
as soon as possible with an occupational lung disease centre.