Effects of the combined exposure to chemicals and unusual working hours.

OBJECTIVE: Both exposure to occupational chemicals and to unusual working hours have well documented effects on health. Determination of occupational exposure limits is, however, usually based on chemical-only exposure and assumes an 8-h workday, 5 days/week and a 40-h work week. A significant proportion of the workforce is exposed to chemicals while working in other work schedules. This review thus aimed to synthesize and evaluate the scientific support for a combined effect of unusual working hours and chemical exposure and, if possible, give recommendations for OEL adjustments to account for unusual working hours. METHODS: The search for articles was made as part of the preparation of a report for the Nordic Expert Group for Criteria Documentation of Health Risks from Chemicals. In this report, unusual working hours were categorized as shift work or extended (>8 h) working hours. Inclusion criteria were observational studies in the English language published up to November 2021 in peer-reviewed journals, with explicit metrics of exposure (chemicals and unusual working hours) and of health outcome, and which explicitly tested the association between exposure and outcome. Search engines of seven databases were used. RESULTS: Of the initially 15 400 identified papers, 9 studies published between 1985 and 2021 met the inclusion criteria, 7 of which showed significant associations. Results from a few of the studies, i.e. regarding effects of dust and endotoxin on lung function, effects of acetone on sleep quality and tiredness, effects of carbon disulphide on coronary artery disease and effects of chemicals on spontaneous abortion, suggested more pronounced effects during night shifts compared to during day shifts. DISCUSSION: The reviewed data is considered insufficient to conclude on recommendations for OEL adjustment for shift work. Suggested areas of future studies are mentioned. CONCLUSION: Further studies about the effects of the combined exposure to unusual working hours and chemical exposure are essential for risk assessment, and for recommendation of potential OEL adjustments. What is important about this paper? Effects of chemical agents at the workplace may depend not only on exposure level and duration but also on the time of exposure in relation to the circadian rhythm. This study reviewed the scientific support for a combined effect of unusual working hours and chemical exposure and revealed an obvious need for additional studies regarding the complex interplay of the two different exposures with respect to adverse health effects.

Extended Work Shifts Increase Subjective Pain Complaints Among Cabin Crewmembers, but Not Among Airline Pilots or Healthcare Workers: A Repeated-Measures Study.

OBJECTIVE: To determine whether four consecutive extended work shifts are associated with an increased risk of subjective pain complaints, sleep duration, and sleep disturbances. METHODS: Forty-three healthcare workers, 41 cabin crewmembers, and 18 airline pilots working 4 consecutive extended workdays reported subjective pain complaints and sleep after the 1st and 4th workday. RESULTS: The risk of headache (odds ratio [OR] 21.4, 95% confidence interval [CI] 1.85 to 246.5) and pain in the hands, arms, or wrists (OR 3.78, 95% CI 1.84 to 7.76) increased after workday 4 versus workday 1 in cabin crewmembers. Sleep duration was longer (0.6 to 1.1 hours), and sleep disturbances fewer, the night before the fourth extended workday, compared with before the first workday, in all occupations. CONCLUSIONS: We found no general support for an association between extended work shifts and subjective pain, whereas sleep duration was improved, and sleep disturbances reduced after 4 consecutive extended workdays.

Safety incidents associated with extended working hours. A systematic review and meta-analysis.

OBJECTIVE: We performed a systematic review to assess potential consequences of extended working hours on accidents, near-accidents, safety incidents and injuries (incidents) by considering the overall certainty of evidence. METHODS: We searched five databases systematically (Medline, Embase, PsycINFO, Web of Science, and Proquest Health and safety Science Abstract) and identified 10072 studies published until December 2020. Twenty-two studies met the inclusion criteria. We followed a systematic approach to evaluate risk of bias and synthesize results in a meta-analysis. The certainty of evidence was determined by a modified version of The Grading of Recommendations Assessment, Development and Evaluation (GRADE). RESULTS: Our analyses indicated an association between working >12 hours/day (RR: 1.24, 95%CI: 1.11, 1.40), or working >55 hours/week (RR: 1.24, 95%CI: 0.98, 1.57), and elevated risk of incidents. The certainty of evidence evaluated as low. Weak or no associations were observed for other exposure contrasts: working >8 hours/day (RR: 0.93, 95%CI: 0.72, 1.19), or working overtime (RR: 1.08, 95%CI: 0.75, 1.55), working 41-48 hours/week (RR: 1.02, 95%CI: 0.92, 1.13) or 49-54 hours/week (RR: 1.02, 95%CI: 0.97, 1.07). The certainty of evidence was evaluated as low (very low for 41-48 hours/week). CONCLUSIONS: Daily working hours >12 hours and weekly working hours exceeding 55 hours was associated and increased risk of incidents. The level of evidence was low. Hence, further high-quality research is warranted to elucidate these associations.

Risk of Fatigue Among Airline Crew During 4 Consecutive Days of Flight Duty.

BACKGROUND: Airline crew are being exposed to extended workdays and compressed work periods, with quick returns between duties, implying a heightened physiological and psychological strain that may lead to sleep deprivation and fatigue. The aim of the study was assessment of the effect of an extended day of flight duty and a compressed work week with regard to recovery, cumulative fatigue, and neurobehavioral performance.METHODS: We followed 18 pilots and 41 cabin crewmembers during four consecutive days of flight duty, comprising a total of ≥ 39 h, where the first day was ≥ 10 h. Information on demographics, work characteristics, health status, and physical activity was collected at baseline. Subjects completed logs for the first and fourth workday, including the Samn-Perelli Fatigue Checklist at three time points during these workdays. Two computer-based neurobehavioral tests were completed the evening prior to the first shift, and after the first and the fourth day of the work period.RESULTS: Number of flight sectors during the work period was 10-20. Self-reported fatigue levels increased during the workdays. Neurobehavioral test-scores did not deteriorate. The effects of each additional flight sector during the work period was elevated reaction times (RT) both among cabin crewmembers (B = 5.05 ms, 95% CI 0.6, 9.5) and pilots (B = 4.95 ms, 95% CI 0.4, 9.5). Precision was unaffected.DISCUSSION: Airline pilots and cabin crewmembers seem to obtain satisfactory sleep before and during the period of 4 consecutive days. The association between multiple flight sectors and increased fatigue supports previous findings.Goffeng EM, Wagstaff A, Nordby K-C, Meland A, Goffeng LO, Skare Ø, Lilja D, Lie J-AS. Risk of fatigue among airline crew during 4 consecutive days of flight duty. Aerosp Med Hum Perform. 2019; 90(5):466-474.

Mechanisms of Breast Cancer in Shift Workers: DNA Methylation in Five Core Circadian Genes in Nurses Working Night Shifts.

Shift work has been suggested to be associated with breast cancer risk, and circadian disruption in shift workers is hypothesized as one of the mechanisms of increased cancer risk. There is, however, insufficient molecular evidence supporting this hypothesis. Using the quantitative methodology of pyrosequencing, epigenetic changes in 5-methyl cytosine (5mC) in five circadian genes CLOCK, BMAL1, CRY1, PER1 and PER2 in female nurses working night shift work (278 breast cancer cases, 280 controls) were analyzed. In breast cancer cases, a medium exposure to night work was associated with increased methylation levels of the CLOCK (p=0.050), BMAL1 (p=0.001) and CRY1 (p=0.040) genes, compared with controls. Within the cases, analysis of the effects of shift work on the methylation patterns showed that methylation of CRY1 was lower in those who had worked night shift and had a high exposure (p=0.006) compared with cases that had worked only days. For cases with a medium exposure to night work, an increase in BMAL1 (p=0.003) and PER1 (p=0.035) methylation was observed compared with day working (unexposed) cases. The methylation levels of the five core circadian genes were also analyzed in relation to the estrogen and progesterone receptors status of the tumors in the cases, and no correlations were observed. Furthermore, nineteen polymorphisms in the five circadian genes were assessed for their effects on the methylation levels of the respective genes, but no associations were found. In summary, our data suggest that epigenetic regulation of CLOCK, BMAL1, CRY1 and PER1 may contribute to breast cancer in shift workers.

Mechanisms of breast cancer risk in shift workers: association of telomere shortening with the duration and intensity of night work.

Occupational factors such as shiftwork and especially night work that involves disruption of the circadian rhythm may contribute to increased breast cancer risk. Circadian disruption may also affect telomere length (TL). While short TL generally is associated with increased cancer risk, its association with breast cancer risk is inconclusive. We suggest that working schedules might be an important factor in assessment of effects of TL on breast cancer risk. Moreover, telomere shortening might be a potential mechanism for night work-related breast cancer. In this study, effects of shift work on TL and its association with breast cancer risk were investigated in a nested breast cancer case-control study of Norwegian nurses. TL was assessed by qPCR in DNA from 563 breast cancer patients and 619 controls. Here, we demonstrate that TL is affected by intensive night work schedules, as work with six consecutive night for a period of more than 5 years was associated with decreased telomere lengths (-3.18, 95% CI: -6.46 to -0.58, P = 0.016). Furthermore, telomere shortening is associated with increased breast cancer risk in workers with long periods of consecutive night shifts. Thus, nurses with longer telomere lengths had a lower risk for breast cancer if they had worked more than four (OR: 0.37, 95% CI: 0.16-0.79, P = 0.014) or five (OR: 0.31, 95% CI: 0.10-0.83, P = 0.029) consecutive night shifts for a period of 5 years or more. These data suggest that telomere shortening is associated with the duration and intensity of night work and may be a contributing factor for breast cancer risk among female shift workers.

Breast cancer among nurses: is the intensity of night work related to hormone receptor status?

The aim of this study was to investigate whether night work is related to breast cancer receptor status. The effect of night work on the risk of estrogen receptor- and progesterone receptor-defined breast cancers was evaluated in 513 nurses diagnosed with breast cancer between 1996 and 2007 and in 757 frequency-matched controls, all of whom were selected from a cohort of Norwegian nurses. Odds ratios for the exposure "duration of work with a minimum of 6 consecutive night shifts" were compared for tumor subgroups with respect to the common control group through the use of polytomous logistic regression. Statistically significant associations were observed between breast cancer and work durations of ≥ 5 years with ≥ 6 consecutive night shifts, with the highest risk observed for progesterone receptor-positive tumors (odds ratio = 2.4, 95% confidence interval: 1.3, 4.3; P-trend = 0.01). When the exposure variable was dichotomized (ever/never worked ≥ 6 consecutive night shifts), a borderline statistically significant heterogeneity (P = 0.05) was seen between progesterone receptor-positive and progesterone receptor-negative tumors in postmenopausal women. The association observed between consecutive night shifts and progesterone receptor-positive cancers suggests that progesterone could play an important role in the detrimental effects of night work.

Night work and breast cancer risk among Norwegian nurses: assessment by different exposure metrics.

Associations between night work and breast cancer risk were investigated in a nested case-control study within a cohort of 49,402 Norwegian nurses. A total of 699 (74%) of the live cases diagnosed in 1990-2007 and 895 (65%) controls, cancer free at the time of sampling, were interviewed about work history and potential risk factors. The odds ratios for risk of breast cancer in relation to different exposure metrics were estimated by multivariate unconditional logistic regression models. No increase of risk was found after long duration of work by nurses working ≥3 night shifts per month. Small, nonsignificantly increased risks were observed for exposure to ≥30 years in hospitals or other institutions (odds ratio (OR) = 1.1), ≥12 years in schedules including night work (OR = 1.3), ≥1,007 night shifts during the lifetime (OR = 1.2), and lifetime average number of ≥4 night shifts per month (OR = 1.2). Nonsignificantly increased risks of breast cancer were observed in nurses who worked ≥5 years with ≥4 (OR = 1.4) and ≥5 (OR = 1.6) consecutive night shifts. Significantly increased risks were seen in nurses who worked ≥5 years with ≥6 consecutive night shifts (OR = 1.8, 95% confidence interval: 1.1, 2.8). The results suggest that risk may be related to number of consecutive night shifts.

Ionizing radiation exposure and cancer risk among Norwegian nurses.

The influence of occupational exposure to ionizing radiation on risk of radiation-related cancers was studied among Norwegian nurses. A cohort of 43 316 nurses who graduated between 1914 and 1984, and were registered by the Norwegian Board of Health's registry of nurses, was followed up from 1953 through 2002 by linkage to the Norwegian Cancer Registry by unique personal identification numbers. Indicators of radiation exposure were developed from data on work history. Internal analyses were performed with Poisson regression, according to time since first potential radiation exposure, duration of exposure, and period of first exposure, using unexposed nurses as reference group. No clear association was found between exposure to ionizing radiation and cancers of the breast, thyroid, ovary, or leukemia, malignant melanoma or other skin cancer. Increased risk of lung cancer was found in the subgroups of nurses first exposed after 1950 (rate ratio=1.47, 95% confidence interval: 0.97-2.23, 26 cases), and in nurses with less than 20 years since first exposure (rate ratio=3.41, 95% confidence interval: 1.67-6.99, 9 cases), but the most likely explanation was confounding by smoking. No firm evidence that nurses potentially exposed to ionizing radiation had increased risk of radiation-related cancer was found.

Cancer risk among 43000 Norwegian nurses.

OBJECTIVES: This study evaluated the influence of occupational exposure on cancer risk among female Norwegian nurses. METHODS: A historical prospective cohort study was performed. The cohort was established from the Norwegian Board of Health's registry of nurses and included women who graduated from a nursing school before 1985. The cohort (N=43 316) was linked to the Cancer Registry of Norway. The observed number of cases was compared with the expected number on the basis of national rates. Time since first employment, period of first employment, and duration of employment were used as indicators of exposure. Poisson regression analyses were used for internal comparisons, adjusting for age, period, and fertility. RESULTS: The nurses were followed over 1473931 person-years. During the follow-up (1953-2002), 6193 cancer cases were observed. The standardized incidence ratio (SIR) for all cancers combined was close to unity. Significantly lower risks were found for cancers with a known association with alcohol and tobacco consumption and sexual activity. A significantly increased risk was found for breast cancer (SIR 1.14, 95% confidence interval (95% CI) 1.09-1.19), ovarian cancer (SIR 1.14, 95% CI 1.04-1.25), malignant melanoma (SIR 1.15, 95% CI 1.04-1.28), and borderline significant risk appeared for other skin cancer (SIR 1.12, 95% CI 0.98-1.29). A positive trend for increasing time since first exposure was found for breast cancer and malignant melanoma. CONCLUSIONS: The results indicate an association between working as a nurse and an increased risk of breast cancer and malignant melanoma. Decreased risks, found for several cancers, indicate favorable lifestyle habits among nurses.

Breast cancer and night work among Norwegian nurses.

OBJECTIVE: Previous studies have suggested an association between breast cancer and night work. We evaluated the relationship among Norwegian nurses. METHODS: A case-control study, nested within a cohort of 44,835 nurses educated between 1914 and 1980 was performed, based on a registry of all Norwegian nurses. Four controls were individually matched by year of birth to each of 537 breast cancer cases that occurred during the period 1960-1982. The reconstruction of work history and number of years with night work for each nurse was based on information from the nurse registry, and data from three censuses. We used conditional logistic regression to calculate odds ratios (ORs) and 95% confidence intervals (CIs), adjusted for total duration of work as a nurse and parity. All statistical tests were two-sided. RESULTS: The adjusted OR of breast cancer among nurses who worked nights for 30 or more years was 2.21 (CI 1.10-4.45) compared with those who did not work nights after graduation from nursing school (p(trend) = 0.01). CONCLUSION: Our results are in accordance with previous studies that find an association between night work and breast cancer risk among women.