End-stage renal disease (ESRD) has a considerable human and social cost. In France, 52 000 patients are treated, with an annual cost of €1.7 billion. Glomerulonephritis are the third leading cause of ESRD, affecting 13% of the cases, after hypertension (21%) and diabetes (20%).1 There are few well-established risk factors of ESRD, but both human and experimental studies suggest that many chemicals can affect the kidney.2 Of these chemicals, the role of organic solvents in chronic kidney diseases, particularly chronic glomerulonephritis, has long been debated.3 Our first findings, based on a case-control study, did not favour an effect of solvent exposure in glomerulonephritis incidence, but rather suggested a role in the progression to ESRD.4 Using an appropriate cohort study design, we recently showed that this exposure was indeed associated with faster progression to ESRD in patients with IgA and membranous glomerulonephritis.5 Solvents, however, include hundreds of chemicals used in a wide variety of products or processes. About 15% of French workers are exposed.6 The identification of specific solvents and exposed job categories at risk would improve intervention to prevent or delay chronic glomerulonephritis progression to ESRD in the occupational setting.

Very few case-control studies investigated the links between specific solvents or solvent-containing products and the risk for chronic or end-stage kidney disease. Oxygenated and aromatic solvents were associated with chronic renal failure in two studies,4 7 and cleaning/degreasing agents with ESRD in two others.8 9 A number of cross-sectional studies of early markers of renal injury were carried out in workers exposed to aromatic or halogenated solvents without yielding conclusive evidence of their nephrotoxicity.10^–14 Only one occupational cohort study examined the association between hydrocarbon exposure and ESRD incidence, showing significant increased risk with chlorinated solvents and JP4 gasoline in aircraft workers.15

Using data from the GN-PROGRESS cohort study, including a large number of new patients with primary glomerulonephritis, we therefore systematically investigated the risk of progression to ESRD, by solvent-exposed job category, type of solvent-containing products, and by solvent or solvent family.

MATERIALS AND METHODS

Study population

The design of the GN-PROGRESS retrospective cohort study has been described in detail elsewhere.5 Briefly, all new Caucasian adult patients with biopsy proven IgA nephropathy (IgAN), membranous nephropathy and focal and segmental glomerulosclerosis, diagnosed between January 1994 and June 2001 in 11 nephrology departments in the Paris area (see Appendix) were invited to participate in the study between January 2002 and March 2004. The diagnosis and primary nature of all glomerulonephritis were confirmed by expert nephrologists and pathologists. Of 538 eligible non-ESRD cases, 18 had died before the study started, 91 could not be traced despite intensive search, 7 refused to participate, 82 were unable to come to a nephrology outpatient clinic to complete the study interview and blood testing mainly because they had moved far from the Paris region, and 2 had poor exposure data, leaving 338 patients including 194 patients with IgAN, 75 with membranous nephropathy and 69 with focal and segmental glomerulosclerosis. In this analysis, we focused on the 269 patients with either IgAN or membranous nephropathy, in whom we previously described an excess risk of ESRD with overall solvent exposure.5

Exposure

On the day of enrolment in the study, patients were asked about their lifetime occupational activities by trained interviewers using a detailed questionnaire to detect solvent exposure as previously described.4 5 For each job period—defined as any period of continuous employment of more than six months in a given job category—we recorded job title, economic activity, period and duration. Participants were asked about any contact with products or processes involving solvents using a detailed list of commonly named items—that is, paints, varnishes, adhesives, printing inks, degreasing and cleaning agents, paint stripping, aerosols and pesticides, chemical and pharmaceutical processes, oil extraction, and petroleum products including fuel, kerosene, gasoline, gas oil or crude oil. Working conditions and frequency of use (every day, at least once a weeķ occasionally) were also recorded. Occupations were coded according to the International Standard Classification of Occupations (ISCO).16 Job periods were reviewed—blind with respect to patient renal function stage—by two industrial hygienists5 who first determined whether reported products were actually solvents on the basis of the context of use including industrial activity, employment period, tasks and working conditions. This step permitted the exclusion of several solvent-free products such as the so-called “glue” reported by construction workers which was actually cement, or some cleaning agents which were actually detergents. Secondly, they identified the types of solvent using a list of the 30 most common ones grouped in five large chemical families: halogenated, aliphatic, aromatic or mixture of both aliphatic and aromatic, oxygenated solvents. For each substance, the experts evaluated the exposure intensity (low, moderate or high) and frequency (occasional (less than once a week) or regular (once a week) subdivided as <2, 2–20, and >20 h/week). These were further combined to define exposure in both a sensitive and more specific way as follows: any exposure level for subjects who had experienced exposure to solvents at any intensity and at any frequency; high exposure level for subjects who had been moderately or highly exposed for more than two hours per week.

Information about smoking habits and alcohol consumption was also obtained by interview. Information on known risk factors for glomerulonephritis progression such as hypertension, proteinuria and serum creatinine level was collected from medical records. Hypertension was defined as a blood pressure >140/90 mm/Hg or antihypertensive treatment. To evaluate renal function at baseline, we estimated glomerular filtration rate (GFR in ml/min/1.73 m²), using the abbreviated Modification of Diet in Renal Disease Study equation based on serum creatinine, age and gender.17 This information was available for all eligible patients with IgAN or membranous nephropathy whether they were participants or non-participants in the study. The occupation at the time of diagnosis or the last occupation in those who were retired was available from medical records for 48% of the latter. Using these data and similar data from the questionnaire for participants, we defined social class as a dummy variable as follows: blue collars, including service, agricultural and production workers—that is, ISCO two digit codes ⩾53—versus white collars including all other job categories—that is, ISCO two digit codes <53.

Outcome

The studied outcome was ESRD, defined according to the classification from the National Kidney Foundation as chronic kidney disease stage 5—that is, either an estimated GFR <15 ml/min/1.73 m² or the start of renal replacement therapy (dialysis or preemptive graft).18 GFR was estimated from serum creatinine measured in all patients at the time of interview, except those who were on dialysis or leaving with a functioning graft. Events were registered over a mean follow-up of five years (median 4.6 years; interquartile range 2.6–7.1 years).

Statistical analyses

IgAN and membranous nephropathy participants’ baseline characteristics were described and compared with those of non-participant eligible patients. To study the progression of glomerulonephritis to ESRD, the time to the event is important, not simply whether or not the event occurs. Therefore, to analyse the relations between solvent exposure and ESRD risk, we used the Cox proportional hazards models adjusted for well-established risk factors of disease progression: age, gender, histological type, proteinuria and hypertension. As solvent exposure is strongly related to social class, we also adjusted for this variable. First, we studied the hazard ratios and 95% confidence intervals (95% CI) of ESRD associated with solvent-exposed job categories. Second, we estimated the hazard ratio associated with exposure to solvent-containing products. Finally, we analysed ESRD risk associated with each of the five solvent families as well as with specific solvents. These analyses were carried out for all job categories, or products, or chemicals with at least five exposed patients. The reference group was that of never exposed patients, which was thus the same in all the above analyses. Bonferroni correction for multiple comparisons was applied by adjusting p values.19 The corrected p values were the p values estimated by models multiplied by the number of tests within each subgroup of job-titles, products, chemical families or substances.

The study protocol was approved by the relevant institutional review board in compliance with French law (Paris Saint Louis Hospital, CCPPRB, decision P001104, 2001). All patients gave written informed consent before enrolment in the study.

RESULTS

Cohort description

Among participants, three patients out of four were men and 35% were blue-collar (table 1). More than a half were hypertensive, about a third had nephrotic range proteinuria, and more than a third had chronic renal failure (GFR <60 ml/min/1.73 m²) at the time of renal biopsy. The participants did not significantly differ from their 150 non-participant counterparts with respect to baseline characteristics as listed in table 1.

Hazard ratios of progression to end-stage renal disease by solvent-exposed job category

Patients reported a mean of three job periods, 18% of which were classified as exposed to solvents by the experts. The distribution of jobs held at least once by the patients are presented in table 2.

The number of these jobs is higher than that of the patients because most patients had more than one job, but it is lower than that of job periods because a patient may have had more than one job period in a given subcategory. Nearly half of the patients in production and transport job categories were classified as exposed to solvents, and one fourth of those in professional and technical job categories. In all other job categories, solvent exposure was scarce. Seventy two per cent of the patients (194) reported no exposed jobs, 30% of whom had been production workers. No significant excess risk of progression to ESRD was observed in glomerulonephritis patients with at least one exposed professional and technical job as compared to those never exposed (table 3). In contrast, this risk was nearly three times higher in patients with at least one exposed production and transport job, independent of age, gender, social class, and kidney risk factors for progression. In that category, significant hazard ratios before Bonferroni correction were observed in those who were machinery fitters, machine assemblers and plumbers or welders, but none of them reached statistical significance after correction (10 exposed job-categories tested).

Hazard ratios of progression to end-stage renal disease by type of solvent-containing product

Paints, degreasing and cleaning agents, and petroleum products were the most common types of reported solvent-containing products (table 4).

Printing inks and petroleum products were significantly associated with an excess risk of progression to ESRD, after controlling for potential confounders and the multiple tests. The elevated adjusted hazard ratio observed for degreasing and cleaning agents was no longer significant after Bonferroni correction, as well as those of other reported products.

Hazard ratios of progression to end-stage renal disease by solvent or solvent family

Aliphatic, oxygenated and halogenated solvents were the most frequent types of chemicals coded by the experts. Solvent families associated with the highest risk of progression to ESRD were aromatic and mixtures of aromatic and aliphatic solvents (table 5). Among these two families, the chemicals most strongly associated with ESRD risk were toluene, xylene and gasoline/fuel/gas-oil. We also observed an excess risk for any exposure level to methylene chloride as well as for high exposure to ketones. In contrast, no significant association was observed for other halogenated or oxygenated solvents, and for aliphatic solvents. With a few exceptions, hazard ratios tended to be stronger when the analysis was restricted to high solvent exposure levels.

DISCUSSION

The prospective study of solvent-related renal risk faces two major obstacles: the low incidence of ESRD, about 130 per million population in France,1 and the wide variety of potential solvents to be tested. Moreover, because, as we have previously shown,5 these substances tend to hasten renal function decline rather than to induce chronic kidney disease, the classic occupational cohort study may not be the best design. Using an appropriate study design based on a large cohort of patients with biopsy-proven glomerulonephritis, we obtained new insights into the types of solvents most strongly associated with progression to ESRD. We found that aromatic hydrocarbons (toluene and xylene), mixtures of aromatic and aliphatic hydrocarbons, ketones and possibly methylene chloride were associated with the highest risks. Among solvent-containing products, the highest risks were consistently related to inks, cleaning or degreasing agents and petroleum products; among solvent-exposed job categories, risks were found for machinery fitters, machine assemblers, plumbers and welders. These findings have straightforward implications for the prevention of solvent-related nephrotoxicity in the workplace.

Assessment of solvent exposure is a critical issue in occupational epidemiology. Studying renal risk by chemical or chemical family is the most relevant approach with respect to aetiology and underlying biological mechanisms. These, however, are most often unknown to exposed workers, and can therefore hardly be later reported whatever the quality of questionnaires and interviewers. To overcome these difficulties, we developed a specific questionnaire in which patients are asked about solvent exposure as they are commonly known to workers. Using this information, together with that on working conditions, experts can assign exposure from three angles: solvent-exposed job categories, solvent-containing products, and types of solvents or solvent families at risk. We analysed solvent-exposed occupations instead of occupations per se—in contrast with most classic epidemiology, because we focused on solvent-related renal injury

Comparison of our results with others is limited by the scarcity of available data and differences in study design. Only two case-control studies analysed the role of solvent families in association with primary glomerulonephritis or chronic renal failure.4 7 In the first one, aromatic solvents were significantly associated with an increased odds ratio of IgAN, and in the other one, they were associated with a non-significant doubling of all-cause chronic renal failure risk, but that study may have lacked power. The potential nephrotoxicity of aromatic solvents was also studied in several cross-sectional surveys, based on early biomarkers of glomerular and/or tubular injuries.13 Some showed an increase in the urinary excretion of tubular markers in toluene and/or xylene exposed workers,10 12 20 but no increase in albuminuria.10 In our study, exposure to toluene and xylene was strongly associated with the risk of progression to ESRD. Regarding styrene, both positive20 21 and negative results11 20 22 were found. Only one patient in our cohort reported a job in the rubber and plastic industry with heavy styrene exposure. He rapidly progressed to ESRD within five years after diagnosis, but we cannot draw any conclusion from this single observation. With respect to oxygenated solvents, our finding of an excess risk with ketones is consistent with that from Nuyts’s case-control study.4 7 In a recent occupational cohort study in US aircraft workers, the authors pointed to the risk of all-cause ESRD associated with trichloroethylene, 1,1,1-trichlorethane, methylene chloride, and JP4 gasoline.15 The latter two were also at risk in our study. Finally, the higher albuminuria rate observed in refinery workers exposed to mixtures of aliphatic and aromatic solvents as compared to controls23 is consistent with the risk related here to gasoline, fuel or gas-oil.

In two case-control studies, cleaning/degreasing agents and petroleum products were found to be associated with all-cause ESRD8 9 and primary glomerulonephritis,9 as we also did for the progression to ESRD. As these solvents were very frequently reported in machinery fitters and assemblers as well as in plumbers/welders, it was not surprising to find excess risks for these job categories. In contrast, although painters were once identified as a group at high risk,24 we did not observe excess risks either for painters or for the use of paints, lacquers and varnishes, which was also the case in the study by Steenland et al.8

Experimental evidence of solvent-induced accelerated renal function decline is limited for several reasons. First, most toxicity studies focused on acute effects of solvents on renal tubules and were not designed to assess chronic nephrotoxicity. Second, experimental models are often unable to mimic exposure conditions in the workplace, making the extrapolation of animal data to human even more difficult. Third, there are very few animal species and strains that are suitable to test the hypothesis that solvents would aggravate rather than cause chronic kidney disease. This was performed, however, in one study designed specifically to assess the role of styrene in the progression of Adriamycin nephrosis in female Sprague-Dawley rats, a well-established model of glomerulopathy.25 It showed that co-exposure to styrene and adriamycin increased the urinary excretion of both albumin and low molecular weight proteins, and was also associated with higher interstitial fibrosis score and greater cellular infiltrates as compared with adriamycin treatment alone. Although limited to a single solvent, this experiment provides some evidence of the role of solvents in kidney disease progression through several potential pathogenic mechanisms including enhanced proteinuria and tubulointerstitial damage. Further experimental studies are, nevertheless, needed to assess other types of solvent.

This study has three limitations. Despite considering all incident patients over a six-year period from all major nephrology centres in the Paris area, we identified only 419 eligible IgAN and membranous nephropathy cases, of whom two thirds were interviewed. Participants did not significantly differ from non-participants with respect to major baseline determinants of glomerulonephritis progression—that is, hypertension, proteinuria and GFR. Neither did they differ with respect to social class, a characteristic strongly related to solvent exposure, but this information recorded from medical records in non-participants was poor. Selection bias as a result of non-participation, therefore, cannot be completely ruled out. Another drawback was the study’s lack of power, which prevented us from analysing several subgroups of solvents or job categories because of our small sample size. That, together with the need for Bonferroni correction (a very conservative method), may have reduced our chance of finding more significant associations, particularly regarding job categories and specific solvents. Retrospective exposure assessment is a critical issue in this type of study. Using a highly detailed and structured questionnaire and blind evaluation of exposures by experts, however, has proved to be reliable.26 We used it in a previous case-control study,4 and it is worth noting that we obtained similar results, providing indirect evidence of reproducibility of the assessment method. A possible weakness of this method, however, is that it strongly relies on individual recall of past exposures. Although most interviews were carried out blind with respect to the patient’s glomerulonephritis stage, this was not possible to achieve for some patients who were seen at the dialysis centre. Recall bias therefore may have contributed to our findings, if patients without ESRD tended to under-report exposures in comparison with those with ESRD or vice versa (if the latter tended to over-report exposures). The specificity of ESRD risk, with respect to solvent types, however, argues against differential bias. Such bias would have indeed increased all hazard ratios independent of the type of solvents, which we did not observe. Nevertheless, the expert assessment was sometimes based on poor information from patient questionnaires but blind with respect to the patient’s glomerulonephritis stage, resulting in possible non-differential misclassification of exposure. Finally, social class may have been a potential confounder for the observed associations, because of its links with both chronic renal failure and solvent exposure,27 but they were not altered after adjustment.

In conclusion, this study highlighted the potential role of toluene and xylene, some petroleum products (gasoline, fuel or gas-oil), ketones and possibly methylene chloride in the progression of chronic kidney disease to ESRD. Further epidemiological studies, however, are needed to confirm these associations and investigate other solvents, such as styrene. It would also be worth studying the effect of these solvents in adequate animal models of renal disease progression in order to better understand the underlying pathogenic mechanisms. Because glomerulonephritis is a major cause of ESRD, interventions to promote screening for proteinuria in solvent exposed workers and discontinuation of exposure in those who are identified with glomerulonephritis at an early stage may prevent or slow the progression to ESRD. Strong interaction between nephrologists and occupational physicians is also important to improve the early detection of renal injury in exposed workers and to manage the care of those diagnosed with glomerulonephritis.