Some pleural effusions labeled as idiopathic could be produced by the inhalation of silica

Exposure to silica nanoparticles has been associated with pleural effusions (PEs) in animal models and case series. We hypothesized that some PEs labelled as "idiopathic" could, in fact, be secondary to inhalation of silica.

A retrospective case control study was designed utilizing a prospectively maintained pleural database. Cases, represented by idiopathic PEs, were matched by age and gender to control patients who had been diagnosed with malignant, cardiac, or infectious PEs. A survey consisting of questions about occupational life and possibility of silica inhalation was conducted. In a subgroup of patients, pleural fluid concentrations of silica were quantified by plasma atomic emission spectrometry analysis. Also, the pleural biopsy of a silica-exposed case was subjected to an energy dispersive X-ray spectroscopy (EDX) to identify the mineral, the size of which was determined by electron microscopy.

A total of 118 patients (59 cases and 59 controls) completed the survey. There were 25 (42%, 95% CI 31-55%) and 13 (22%, 95% CI 13-34%) silica-exposed workers in case and control groups, respectively. The exposure attributable fraction was 0.62 (95% CI 0.14-0.83). Four of eight exposed cases showed detectable levels of silica in the pleural fluid (mean 2.37 mg/L), as compared to none of 16 tested controls. Silica nanoparticles of 6-7 nm were identified in the pleural biopsy of an exposed case patient.

It is plausible that some idiopathic PEs could actually be caused by occupational silica inhalation.