Part 1
This is to our knowledge the first scientific study to address pulmonary health among offshore drill floor workers. Lung function measurements at baseline were carried out at an earlier time of the day than at follow-up, due to the time schedule of the helicopter transport to and from the oil rigs. After correction for the examination time difference, FEV1 declined statistically significantly across the 14-day work period among the drill floor workers, but not among the referents. Contrary to the á priori hypothesis, the decline was most pronounced, when there was less active drilling and thereby less mud circulation.
Diurnal variation in pulmonary function is well-known (Guberan et al.
1969; Hetzel and Clark
1980; Borsboom et al.
1999; Spengler and Shea
2000). In particular, the study by Spengler and Shea (
2000) that was specifically designed to assess diurnal variation, showed a substantial increase in FEV
1 from 8:00 a.m. to 10:00 a.m. (approximately 200 mL) and a gradual decline from 14:00 p.m. to 18:00 p.m. Also, Borsboom et al. (
1999) reported substantial improvement in FEV
1 from 9.00 a.m. to around noon, followed by a gradual decrease at 8:00 p.m. Based on their estimation of FEV
1 in a subject aged 44.5 years, the difference in FEV
1 among the persons examined at 9 a.m. or 11 a.m. would be around 70 mL. The procedure of helicopters transporting crews offshore before returning the old crews onshore may have resulted in a systematic difference between baseline and follow-up in the time of lung function measurements and thus affected the amount of decline in pulmonary function between baseline and follow-up.
There are no standard methods for correction of diurnal pulmonary variation. As shown by Spengler and Shea (
2000), the diurnal variations in FEV
1 and FVC do not follow a simple cosinus/sinus curve, or a mathematical curve easily described in few parameters. To estimate the diurnal variations of our data, we could have used an additive mixed model approach, where the diurnal variation is described by a smooth flexible curve. However, we had too few observations and too much variability in the data to possibly estimate such a flexible curve with precision. Therefore, we used the corrections for circadian rhythms given by Spengler and Shea (
2000). Because of the substantial improvement in FEV1 in the morning, it is important to consider diurnal variation in epidemiological studies. They estimated the peak to trough of mean circadian change of FEV
1 to 3.5%, while Borsboom et al. (
1999) estimated a mean circadian change in FEV
1 of 2.8% (86 mL). These estimates are well within the mean decline in FEV
1 of 60 and 70 mL among the drill floor workers and referents, respectively, before correction for diurnal variation. After correction, the drill floor workers had a statistically significant mean FEV
1 decline of 90 mL across the 14-day work period, in contrast to a non-significant mean decline of 20 mL among the referents. The difference in FEV
1 decline between drill floor workers and referents did not reach statistical significance (
p = 0.086), but this may be due to the limited sample size. However, a slight occupational exposure cannot be excluded among the referents. Such exposure may have reduced the differences in pulmonary function between the two groups. Regression analysis suggested that the decline in FEV
1 was substantially smaller, when active drilling was performed than when no active drilling took place (Fig.
2). This could indicate that decreased pulmonary function among the drill floor workers was not associated with exposure to oil mist from circulating mud. This suggestion is further substantiated by their low exposure of oil mist and oil vapor (median concentrations of 0.18 and 14 mg/m
3, respectively), when compared to national occupational exposure limits of 0.6 and 30 mg/m
3 (12 h TWA). Moreover, the available data suggest that active drilling occurred for around 7 days on average. Confidential drilling data were not available from all the companies, which is a limitation of the study.
The drill floor workers were also exposed to mud components other than oil (median 0.14 mg/m
3). Some mud components are known irritants to the skin and eyes, and both oil-based and water-based muds are alkaline solutions (Hansen et al.
1991; Saeed et al.
1997). The process of drilling consists of different phases, and one of the involved drilling companies has estimated that the mud pumps run for approximately 25% of the time during drilling operations (personal information). In periods without circulation of mud, the drill floor workers are usually not working in the shaker area. Work tasks then typically involves running in or pulling out the drill stem, maintenance and assembling of drilling equipment. Pulling out the mud-filled drill stems leads to variable degrees of mud spills at the drill floor, depending on equipment design. Cleaning up spills of mud, oil and grease are frequent work tasks for drill floor workers during non-drilling periods. The contaminated surfaces are usually sprayed with an alkaline detergent, before the solution is removed by pressure washing. As shown in shaker rooms, pressure washing may lead to aerosol generation from the substances that are removed by the cleaning process (Kirkhus et al.
2015). The cleaning agents used are described as irritating to the airways. It is imaginable that such exposures may impact pulmonary function during non-drilling periods. Potential chemical exposures during non-drilling periods were not assessed, but should be addressed in future studies. Health examinations were carried out at the heliport at Sola airport that is a hub for transport of personnel to and from the offshore sector. Many participants live far away from this hub, and many of them travel by plane between their home and the heliport. Often, the time between helicopter transport and commuting flights is limited. We assume that this has contributed to the loss to follow-up of 26 subjects. These subjects did not differ from the other participants with respect to important background variables. Thus, this loss to follow-up has most likely not systematically distorted the results, but resulted in a lower study power than originally planned. However, the use of linear mixed models should partly account for this loss of power, as incomplete observations can also be included in such analyses.
Offshore workers have their health examined every second year to assess their fitness for work. This could suggest that subjects with a disease may stop working offshore. This potential selection of particularly healthy workers is not likely to bias the results of Part 1 of the study, as pulmonary function decline across a 14-day work period was the outcome. Moreover, the potential dilution of the reference group with former drill floor workers is small, because the highly specialized knowledge of the drill team workers are less required in other departments of the rig. The exposed workers have long working hours, but potential fatigue should be evenly distributed in both the groups. There were also no signs of the impact of fatigue in the study of Spengler and Shea (
2000). It may also be possible that helicopter transportation for up to 2 h prior to the second examination may have had some impact on pulmonary function, but this would eventually affect the results equally in the two groups.
Part 2
Examinations with HRCT showed minor signs of pulmonary fibrosis in one subject only. The former drill floor workers had been exposed during a period with substantially higher exposure to oil mist, than what had been the case for the participants in Part 1 of the study. An arithmetic mean concentration of 4.3 mg/m
3 measured by personal sampling was reported for the period 1989–1997 (Steinsvåg et al.
2006).
It has been suggested that exposure to oil mist may cause pulmonary fibrosis in cable workers exposed to oil mist levels of 0.5–1.0 mg/m
3, and in animals exposed to cable oils (Skyberg et al.
1986,
1990,
1992), but only minor signs of pulmonary fibrosis were detected among the 57 examined subjects. One subject with subpleural reticular pattern without cysts and superimposed ground glass opacities representing fine intralobular fibrosis with an overall extent of 5–10% had additional pleural plaques and small rounded atelectasis and parenchymal bands, indicative of previous asbestos exposure and asbestosis. This does not exclude the possibility that fibrosis may occur after oil mist exposure, but at such high exposure levels, it is at least a rare event.
Signs of small airways disease were present in 33% of the drill floor workers participating in Part 2 of the study. A study of air trapping on HRCT images in asymptomatic subjects showed a prevalence of 52%, while the prevalence for subjects aged 51–60 years was 65% (Lee et al.
2000). Thus, air trapping is common, and cannot be attributed to oil mist exposure among the drill floor workers. It should, however, be noted that only employees who had worked in the three larger companies participated in Part 2 of the study. It is possible that the exposure levels of the drill floors in these companies may have been low compared to the other companies, which could result in an underestimation of a potential development of pulmonary fibrosis. However, we do not have any such information. Also, of the 118 eligible drill floor workers, only 57 participated. Disease status, including pulmonary fibrosis, is unknown for the non-participating subjects.
In conclusion, after correction for diurnal variation in pulmonary function, the drill floor workers experienced a decline in FEV1 across the 14-day work period offshore. There were, however, no indications of an association with oil mist exposure. The possible impact of exposure of the other airborne contaminants should be further explored. No indications of interstitial lung diseases related to oil mist exposure in drill floor workers were observed.