Introduction

Globally, an estimated 400,000 drowning deaths occur each year. [1] In Canada, drowning is the third leading overall cause of unintentional death before the age of 60, averaging about 3 fatalities each week in Ontario (Canada’s largest province). [2] Drowning rates in Canada are similar to those of the United States, but generally lower than rates in Africa or Asia. [1] Drowning deaths are a devastating event that commonly affects young healthy individuals who might have otherwise lived a long and healthy life. Drowning deaths also carry a significant economic burden on the Canadian health care system accounting for about $106 million (Canadian dollars) in costs in 2004 [3] Despite ongoing drowning prevention strategies, the number of drowning deaths have been increasing in Canada since 2004. [4].

Most drowning deaths are preventable. [5] Ethanol use, reckless behaviour, and inadequate water safety instructions are all major modifiable risk factors. [2], [5]–[7] Personal flotation devices can save lives, as can fences surrounding swimming pools, lifeguards and swim ‘buddies’. [8]–[13] In all cases, saving lives requires that people recognize risks and avoid a false sense of security around aquatic environments. Popular media, both old and new, portray drowning events taking place during cold, stormy, dark weather. [14]–[17] Further, the Lifesaving Society of Canada as well as the Centers for Disease Control and Prevention in the United States focus on the risk of drowning during inclement weather. [18]–[19] This portrayal has the potential to create a false sense of security when people engage in aquatic activities on warm days. Few rigorous studies support the association of weather with drowning risk, and the available studies lack a control group, pertain to occupational settings, or apply only during natural disasters [20]–[22].

Most drowning deaths in Canada occur during the summer months and most occur during recreational activities. [2] We hypothesized, therefore, that hot weather rather than cool stormy weather might be associated with higher rates of drowning. To explore this hypothesis we combined data from the Office of the Chief Coroner with official governmental meteorological records to assess the association of hot weather with drowning risk. This approach used a self-matching design to control for access to health care, public health programs, population education, lifestyle patterns and multiple other confounding factors. If hot weather is associated with an increased risk of drowning, a greater awareness of this association might motivate more targeted lifesaving initiatives.

Methods

Inclusion and Exclusion Criteria

Using Coroner’s records, we identified consecutive deaths among Ontario residents aged 1 day and older during the ten year study interval. The study interval up until 2009 was selected because it was the most recent decade available that provided adequate sample size taking into account the lag time for Coroner’s reports. We excluded non-fatal drowning, intentional events (eg, suicide or homicide), deaths among non-residents of Ontario, and cases where the date of drowning was unknown (Figure 1). [25] Non-fatal drowning deaths were excluded since the data were not available at the Office of the Chief Coroner or any other centralized database. Non-residents of Ontario were excluded due to the lack of identifying information necessary for demographic data and medical comorbidities.The date and time of the drowning was determined from the police report where available and the coroner’s report otherwise.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 1. Drowning deaths in the province of Ontario (1999–2009).

https://doi.org/10.1371/journal.pone.0071689.g001

Results

Drowning Cases

We identified 1243 drowning deaths in Ontario during the ten year study, equivalent to about 120 deaths per year for a population of about 12 million people. About 82% were male (n = 1020). The mean age was 40 years and about one quarter were over the age of 55 years (Table 1). The majority occurred in outdoor locations and the majority had not been wearing a personal flotation device. About one in three had been drinking alcohol. Cardiac disease was the most common predisposing medical condition., although most cases had no medical comorbidity (Table 2).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Drowning Characteristics.

https://doi.org/10.1371/journal.pone.0071689.t001

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 2. Location of drowning and medical comorbidities.

https://doi.org/10.1371/journal.pone.0071689.t002

Discussion

Our study is the first to analyze the association between hot weather and drowning risk for over one thousand deaths during ten years in Canada. We found that the number of outdoor drownings increased significantly on days when the maximum temperature exceeds 30°C. Men were specifically prone to drown on hot days compared to cool days. Children were twice as likely to drown on hot days compared to cool days. These findings are unlikely to reflect random chance, differences in water safety knowledge, or the availability of protective equipment. Instead, the data suggest that hot weather rather than cold stormy weather increases the risk of death by drowning.

Previous case series of drowning deaths in the fishing industry and drowning deaths following natural disasters emphasize the hazards of stormy weather. [20]–[22] We do not doubt this risk is real, but highlight that such risk reflects a minority of drowning deaths in Canada. Data from large case series in China and Bangladesh found that drowning deaths increased with temperature elevations. [28]–[29] Our study has formally tested this hypothesis in a four-season climate and our results confirm that hot weather is the greater danger to public health. Further, a recent report from the Office of the Chief Coroner supports our findings that hot weather is a common element in drowning risk [30].

Several reasons might explain why hot temperatures are associated with higher rates of drowning. First, drowning is directly correlated with aquatic exposure. [28], [31]–[32] As temperature rises, therefore, people spend more time around the water. Second, alcohol use is a risk factor for drowning and consumption may increase on hot weather days. [2], [32]–[33] Alcohol negatively affects judgment and swimming ability, perhaps explaining the distinct drowning risk for adult men since men are more likely to be under the influence of alcohol during aquatic activities and engage in risky behaviours. [33] In addition, use of a personal flotation device may decline on warm summer days among all age groups. Although some studies have demonstrated that cardiac mortality rates increase at the extremes of temperature, [34] these studies excluded accidental deaths (e.g., drowning) and focus on the elderly or patients with multiple medical comorbidities. [35]–[36] Since most deaths in our study were young adults with no medical combordities it is unlikely that the observed increased drowning rates are related to underlying trends in cardiac mortality related to hot weather.

Several weaknesses of this study merit emphasis. First, our study design was retrospective and cannot determine causality; in particular, we do not know whether individuals were involved in water-related activities during the control day and thus the observed increased risk may be due to increased aquatic exposure alone. Second, we selected a single control period for study feasibility, whereas additional control periods might have improved the robustness of our findings. Third, we attempted to record hourly weather data when possible, yet hourly meteorological data were not available for all drowning deaths. Since the weather can vary within a given day, we cannot be certain that daily maximum temperature accurately reflected the air temperature during the individual drowning (a second source of measure error that also slants our analysis to the null). Fourth, our data set did not include non-fatal drowning events. [25] Since non-fatal drowning events occur approximately five times more often than drowning deaths, our study substantially underestimates the absolute risks [1], [5].

Some additional limitations of our study also merit consideration. Our study occurred in a high-income country with universal access to health care; hence, our results may not be applicable to low income countries where drowning rates are particularly high. We were not able to control for long-weekends marked by holidays; however, such imbalances may be partially offset by drowning deaths that occurred one week after a long-weekend, since the long-weekend would now become the control interval. We also did not have access to data regarding drowning risk factors from the control interval (e.g., alcohol intake). Also, air temperature may not be the most accurate measurement for “weather”. Instead, cloud coverage, humidity, wind speed, or rainfall might represent further distinctions that influences a person’s behaviour in water-related activities. Studying the effect of water temperature on drowning risk would also be important, but was not feasible for this study as the data is not available from Environment Canada. Our analysis was not powered to analyze multiple air temperature thresholds. Lastly, although indoor drowning deaths were not associated with hot weather, this may be due to a small sample size of indoor drownings and this lack of an association does not necessarily confirm our study hypothesis. Further research is justified to address these limitations.

Our study suggests an association but not necessarily a causal relationship between hot weather and the risk of drowning. For example, hot weather may lead to increased water-related activities, increased alcohol consumption, increased reckless behaviour, or lower use of a personal flotation device. [22] In effect, hot weather alone may not increase drowning risk but may influence behavioural changes that predispose to drowning. These mechanisms, therefore, provide opportunities for diverse drowning prevention initiatives. None of these uncertainties justify a nihilistic approach to water safety.

Strategies for drowning prevention fall into three categories; namely, education, engineering, and enforcement. [37] Although our study does not address the effectiveness of these strategies they are fundamentally important to prevent future drowning deaths. More persons could be educated about the risk of drowning associated with hot weather and, in particular, receive specific warnings about responsible alcohol consumption during hot weather days. A lighter, sleeker inflatable personal flotation device or other technologic improvements might also help to prevent future drowning deaths. Applying stricter legislation for personal flotation device use is another option, as is continued enforcement of alcohol-free watercraft operation. An awareness of risk can lead to thoughtful action whereas a popular misconception can result in misdirected resources and misfortunate consequences.

Acknowledgments

We thank the Office of the Chief Coroner of Ontario (Dr. Barry MacLellan, Dr. Andrew McCallum, Dr. Dan Cass, and June Lindsell) for their support and assistance. The Office of the Chief Coroner had no role in the design, conduct or analysis of this study. We thank Nashwah Taha and Alexandra Stefan for assistance with data collection and Alex Kiss for biostatistical support.