Canada-Wide Distribution of Environmental and Occupational Risk Factors for Urinary Stone Disease: Insights for Equitable Resource Allocation and Fighting Health Disparities

Abstract

Abstract: Objectives: This study aimed to map the distribution of nephrolithiasis’ environmental risk factors (occupational heat and heavy metal exposure and ambient seasonal temperature) and to assess the correlations of these exposures with the best estimates of the reported nephrolithiasis incidence in Canada. Methods: The regional average heat burden was defined as the mean temperature in the hottest three months of the year for 2020, 2021, and 2022. The employment rates in the top five industries with occupational heavy metal (cadmium, lead, and arsenic) and heat exposure were obtained from the Statistics Canada 2021 database. Statistical significance was calculated based on the 95% confidence interval difference from the null hypothesis. Correlation analysis was performed between our rates of nephrolithiasis risk factors and previously published estimates of the stone incidence: kidney stone interventions and acute kidney stone event rates. Results: Lower-latitude provinces had higher overall mean temperatures in 2020 to 2022, with Ontario, Manitoba, and Prince Edward Island having the highest seasonal heat burdens, in this order. Nunavut had the lowest rate of occupational heat exposure, while the remaining regions had similar rates. Yukon, the Northwest Territories, and Nunavut had significantly higher rates of occupational heavy metal exposure compared to the remaining regions. The ambient temperature and occupation heavy metal and heat exposure showed no significant correlation with the estimates of the stone incidence. Conclusions: The occupational heat exposure was relatively similar between regions. Northern Canada had higher occupational heavy metal exposure compared to other regions. Occupational exposures and temperature variations were not associated with the nephrolithiasis incidence in Canada.

1. Introduction

Nephrolithiasis is associated with frequent emergency visits and increased healthcare costs, with a rise in prevalence globally [1]. While health and lifestyle changes account for a large percentage of this increased prevalence, environmental occupational exposures can play a crucial role in nephrolithiasis.

The ambient temperature is a known prognosticator for kidney stone prevalence [2,3]. The increased incidence of nephrolithiasis has been reported in the summer months, also referred to as “stone season” [1,4]. As a result of climate change, Canada showed a 2.1 degrees Celsius (°C) increase in its annual temperature in 2021 from the reference temperatures in 1961–1990 [5]. The up-trending average annual temperatures lead us to question whether adequate resource planning has taken place to mitigate the rising incidence of nephrolithiasis, especially during the summer months.

Exposure to heavy metals, such as cadmium (Cd), lead (Pb), and arsenic (As), poses significant public health risks according to the World Health Organization, and they have been studied regarding their link to nephrolithiasis [2,6,7,8,9].

We evaluated the distribution of environmental nephrolithiasis risk factors, including the ambient temperature, workplace heat stress, and heavy metal (Cd, Pb, and As) occupational exposure, across Canada. Our findings may guide resource planning and targeted public health interventions to ensure equitable health outcomes in Canada.

As a secondary objective, we assessed the correlations among these exposures and the recently reported incidence of nephrolithiasis.

2. Methods

Seasonal temperature changes—Canada is divided into 13 distinct regions, 10 provinces, and 3 territories. The mean daily temperature at the Environment Canada temperature recording stations in each province or territory was averaged to determine the average monthly regional temperature. Subsequently, the temperatures of the 3 warmest months of each year from 2020 to 2022 were averaged to establish the average ambient heat burden in each region.

Occupational heat stress—We identified nine industries with heat stress using worker health information resources including the National Institute for Occupational Safety and Health (NIOSH), the United States Department of Labor, and WorkSafe BC. These were “mining and quarrying”; “electric power generation, transmission, and distribution”; “construction”; “primary metal manufacturing”; “pulp, paper, and paperboard mills”; “chemical manufacturing”; “clay product and refractory manufacturing”; “glass and glass product manufacturing”; “fabricated metal product manufacturing”; and “dry cleaning and laundry services”. The percentage of Canadians exposed to occupational heat stress was calculated using Statistics Canada 2021 data on the number of Canadians (15 years or older) who were employed in the aforementioned industries.

Occupational cadmium, lead, and arsenic—CAREX Canada [10] is a publicly available resource that identifies industries with high heavy metal exposure, as well as the percentage of workers exposed to these heavy metals in these industries. From the Statistics Canada 2021 database, we retrieved the number of Canadians (15 years or older) who were employed in the top 5 industries identified by CAREX Canada. The data for all three heavy metals (cadmium, lead, and arsenic) were combined.

Statistical appraisal of data—Comparative statistics were performed by assessing for overlap between the 95% confidence intervals provided by Statistics Canada. Conservatively, we defined statistical significance between the means as a lack of overlap between the 95% confidence intervals and accepted the null hypothesis otherwise.

A correlation analysis was performed between the rates of the nephrolithiasis risk factors found in this study and the best estimates of the stone incidence using the published incidence of acute kidney stone events or kidney stone interventions (ureteroscopy, percutaneous nephrolithotomy, and extracorporeal shock wave lithotripsy) per 100,000 population, regionalized by province [11]. Of note, the incidence of acute kidney stone events for the province of Quebec and the incidence of kidney stone interventions for Quebec and the three territories (Yukon, Nunavut, Northwest Territories) were not available.

3. Results

The seasonal heat burden over 2020–2022 in Nunavut was the lowest (average 6.4 °C) among the hottest three months of the year. Lower-latitude provinces had higher mean temperatures in 2020 to 2022, with Ontario, Manitoba, and Prince Edward Island having the highest seasonal heat burdens, in this order (Figure 1A).

Nunavut and Nova Scotia had the lowest rates of occupational heat exposure, with only 375 per 10,000 (95% CI 364–408) and 619 per 10,000 (95% CI 596–644) employed in industries with heat stress, respectively (Figure 1B and Figure 2A). The remaining regions had employment rates in heat-stressed industries ranging from 685 per 10,000 (95% CI 652–721) in Newfoundland to 864 per 10,000 (95% CI 767–985) in Yukon.

The combined occupational Cd, Pb, and As exposures were seen at significantly higher rates in Yukon and the Northwest Territories, namely 118 per 10,000 (95% CI 104–137) and 106 per 10,000 (95% CI 98–116), respectively, compared to 57 per 10,000 (95% CI 53–61) to 81 per 10,000 (95% CI 77–86) in the other provinces (Figure 1C and Figure 2B).

Heavy metal and heat occupational exposure, along with the ambient temperature, showed no significant correlation with the resource utilization rates for kidney stone interventions (Pearson coefficients: −0.65, −0.64, and −0.7, respectively) or acute kidney stone event rates (Pearson coefficients: −0.50, 0.07, and 0.32, respectively) (

Table 1. Correlation analysis between kidney stone risk factors and rate of acute care for kidney stone events or rate of kidney stone intervention (ureteroscopy, percutaneous nephrolithotomy, and extracorporeal shock wave lithotripsy). No data from Quebec were available for correlation analysis. Rates of kidney stone intervention in the territories were unavailable.

Kidney Stone Risk Factor	Rate for Resource Use for Any Kidney Stone Intervention in 2013–2018 per 100,000 (95% CI)	Rate of Acute Care for Kidney Stone Events in 2013–2018 per (95% CI) 100,000
Occupational cadmium, lead, or arsenic exposure	−0.65 (−0.92, 0.03)	−0.50 (−0.86, 0.19)
Occupational heat exposure	−0.64 (−0.92, 0.03)	0.07 (−0.58, 0.67)
Overall mean Temp of the warmest months in 2020–2022	−0.73 (−0.92, −0.13)	0.32 (−0.39, 0.79)

).

4. Discussion

The rates of occupational heavy metals were not correlated with acute kidney stone events or the rate of resource use for stone interventions, which suggests that possible confounders likely exist. For example, Nova Scotia has low occupational exposure but high rates of diabetes, hypertension, and obesity [12], which could mask the potential effects of occupational exposure. We could not control for these, or for dietary differences between provinces, with our dataset. Furthermore, the ambient temperature was assessed by necessity by province/territory, but the latitude likely plays a greater role in temperature exposure. For the territories, high occupational exposures and lower ambient temperature risks could also offset one another. Regardless, heavy metals and occupational/ambient temperature exposure do not seem to be the primary drivers of kidney stones in Canada.

The link between cadmium, lead, and arsenic and nephrolithiasis has been previously suggested [7,8,9]. In a 2023 report including 9056 adults from the United States, an increased urinary cadmium concentration was associated with an increased risk of kidney stones [13]. Interestingly, the following three industries were among the top five exposure groups for both cadmium and lead: “architectural and structural metal manufacturing”, “automotive repair and maintenance”, and “commercial and industrial machinery and equipment”. Further studies are necessary to determine whether co-exposure to cadmium and lead has a synergistic effect on the rates of renal stone formation. Increased odds of kidney stones with elevated serum [7] and urine [8] arsenic concentrations have also been documented. The outdoor nature of the top five industries with arsenic exposure and higher ambient temperatures during summer may have also have synergistic effects regarding the development of nephrolithiasis in this population and could be further explored in future studies.

While we captured the seasonal heat burden over 3 years, historical climate records in Canada highlight the effects of climate change, with the summer of 2021 being 1.5 °C warmer than that of 1948 [5]. Climate change will likely intensify the ambient temperatures at the extremes in the coming years and increase the seasonal heat burden [14]. In a predictive correlation analysis of the stone prevalence derived from doctor visits for upper urinary tract stone disease in the United States, it was found that a 4.2% increase in nephrolithiasis prevalence can be expected for every degree Celsius increase in the mean annual temperature [3]. Although these models are hypothetical, they can be of utility in guiding resource allocation for the prevention and mitigation of renal stone disease. The lack of correlation between the seasonal temperature differences and kidney stone incidence in our study suggests that other factors may have a stronger influence in Canada. Seasonal variations have been shown to play a factor in acute kidney stone events in other studies [4,15]. In a retrospective longitudinal study from New Zealand, a 2.8% increase in emergency department visits for acute urinary colic was noted in the summer season [4]. A systematic review by Geraghty et al. in 2017 found 13 studies on the effects of seasonal variations on urolithiasis events, 12 of which showed a statistically significant association between stone events and higher average monthly temperatures [15]. Therefore, the importance of urolithiasis-preventative strategies in the greater global context should not be overlooked, especially given the predicted adverse effects of climate change on health outcomes [16]. We suggest that clinicians reinforce the importance of fluid intake, according to both the region and the average ambient temperature, for optimal stone prevention.

In terms of occupational heat stress, previous studies have reported higher rates of renal stone disease among steel workers and glass manufacturing machinists [17,18]. However, our study noted relatively similar exposure rates in the Canadian context (Figure 1B) and no correlation between occupational heat stress and renal stone events. Our analysis of occupational heat exposure was limited by the unavailability of data on the duration of heat exposure during work hours and the presence of implemented preventative strategies. For instance, mandatory break times to allow for rehydration and the availability of rehydration stations nearby could be examples of existing mitigation strategies in Canada used to offset the effect of occupational heat stress. Nevertheless, the link between occupational heat exposure and kidney stone events in other countries merits further investigation.

Nonetheless, this study is the first, to our knowledge, to assess the distribution of occupational and environmental nephrolithiasis risk factors in Canada and to determine whether these correlate with the incidence of kidney stones.

The results of our study must be considered within the context of a few remaining limitations. First, some temperature-reading stations in Nunavut are in sparsely populated northernmost areas. These stations could inadvertently skew Nunavut’s average temperature calculations downward, making it an imperfect representation of the ambient temperature exposure of the population. Nevertheless, in order to capture the diffusely populated nature of the region, these reading stations were not eliminated. Second, a thorough list of occupational exposures—including mercury, which is a nephrotoxin associated with nephrolithiasis [8]—was hindered by the limited data availability from CAREX Canada.

5. Conclusions

Occupational exposures and temperature variations were not associated with the nephrolithiasis incidence in Canada. Other factors, including comorbidities or diet, may be predominantly driving the kidney stone incidence, or the temperature–stone relationship may be distributed according to other factors, such as the latitude, rather than the province.