atmosphere Article Temperature-Related Mortality in Helsinki Compared to Its Surrounding Region Over Two Decades, with Special Emphasis on Intensive Heatwaves Reija Ruuhela 1,*, Athanasios Votsis 1, Jaakko Kukkonen 2, Kirsti Jylhä 1 , Susanna Kankaanpää 3 and Adriaan Perrels 1 1 Weather and Climate Change Impact Research, Finnish Meteorological Institute, FI-00560 Helsinki, Finland; athanasios.votsis@fmi.fi (A.V.); kirsti.jylha@fmi.fi (K.J.); adriaan.perrels@fmi.fi (A.P.) 2 Atmospheric Composition Research, Finnish Meteorological Institute, FI-00560 Helsinki, Finland; jaakko.kukkonen@fmi.fi 3 Finland Adaptation to Climate Change, City of Helsinki, 00100 Helsinki, Finland; susanna.kankaanpaa@hel.fi * Correspondence: reija.ruuhela@fmi.fi; Tel.: +358-500-424533 Abstract: Urbanization and ongoing climate change increase the exposure of the populations to heat stress, and the urban heat island (UHI) effect may magnify heat-related mortality, especially during heatwaves. We studied temperature-related mortality in the city of Helsinki—with urban and suburban land uses—and in the surrounding Helsinki-Uusimaa hospital district (HUS-H, excluding Helsinki)—with more rural types of land uses—in southern Finland for two decades, 2000–2018. Dependence of the risk of daily all-cause deaths (all-age and 75+ years) on daily mean temperature was modelled using the distributed lag nonlinear model (DLNM). The modelled relationships were applied in assessing deaths attributable to four intensive heatwaves during the study period. The results showed that the heat-related mortality risk was substantially higher in Helsinki than in HUS- H, and the mortality rates attributable to four intensive heatwaves (2003, 2010, 2014 and 2018) were Citation: Ruuhela, R.; Votsis, A.; about 2.5 times higher in Helsinki than in HUS-H. Among the elderly, heat-related risks were also Kukkonen, J.; Jylhä, K.; Kankaanpää, higher in Helsinki, while cold-related risks were higher in the surrounding region. The temperature S.; Perrels, A. Temperature-Related ranges recorded in the fairly coarse resolution gridded datasets were not distinctly different in the two Mortality in Helsinki Compared to Its considered regions. It is therefore probable that the modelling underestimated the actual exposure Surrounding Region Over Two Decades, with Special Emphasis on to the heat stress in Helsinki. We also studied the modifying, short-term impact of air quality on Intensive Heatwaves. Atmosphere the modelled temperature-mortality association in Helsinki; this effect was found to be small. We 2021, 12, 46. https://doi.org/ discuss a need for higher resolution data and modelling the UHI effect, and regional differences in 10.3390/atmos12010046 vulnerability to thermal stress. Received: 30 November 2020 Keywords: temperature-related mortality; urban heat island; heatwave Accepted: 23 December 2020 Published: 30 December 2020 Publisher’s Note: MDPI stays neu- 1. Introduction tral with regard to jurisdictional clai- Ongoing megatrends, such as climate change and urbanization, are projected to glob- ms in published maps and institutio- ally increase the exposure of populations to heat stress. Growing cities and the densification nal affiliations. of cities may lead to a stronger urban heat island (UHI) effect, which may substantially exac- erbate health impacts related to heat stress, especially during heatwaves, while on the other hand, reduce exposure to cold stress [1]. In northern European countries, where severe heat- Copyright: © 2020 by the authors. Li- waves are less frequent than in hotter climates and effective cooling systems in buildings censee MDPI, Basel, Switzerland. are not common, the UHI effect may be responsible for a large share of heat-related health This article is an open access article impacts. In this article, we have modelled temperature–mortality relationships in the city distributed under the terms and con- of Helsinki and its wider surrounding region in southern Finland, the Helsinki-Uusimaa ditions of the Creative Commons At- hospital district. Our main motivations were to assess, whether the temperature-related tribution (CC BY) license (https:// mortality would be higher in the capital, compared to the surrounding region, and whether creativecommons.org/licenses/by/ the UHI would have an impact on the temperature-related mortality in Helsinki. 4.0/). Atmosphere 2021, 12, 46. https://doi.org/10.3390/atmos12010046 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 46 2 of 13 Temperature-mortality relationships are typically found to be U- or J-shaped, and according to earlier studies, this relation applies for Finland, as well [2–4]. The major heatwaves in recent decades—in 2003, 2010, 2014 and 2018—have been evaluated to have each caused about 200 to 400 excess deaths in Finland [5,6]. Several multicountry studies on temperature-related mortality have used mortality and climate data from Helsinki, as well [7–11]. In these studies, Helsinki has been considered as a representative city for the northern European climate or representative for Finland in general, without specific discussion on the role of UHI in temperature-related mortality. However, the UHI may lead to temperatures that are several degrees higher in cities than in surrounding rural areas, and consequently magnifies the exposure of the population to heat stress. Urban climate studies have been concentrated on mapping or modelling the spatial distribution of the UHI with an aim to understand how characteristics of the cities modify the UHI in single cities. It has been concluded that the UHI effect varies with respect to the city size, land use, topographic factors, vegetation, urbanization and industrialization, season of the year and time of day, as well as with respect to the prevailing meteorological conditions [12,13]. Recently some multicity studies have studied the exposure to heat waves and cold waves in urban climates across Europe [14], as well as the impact of urban characteristics on the vulnerability of the population to heat and cold stress [15]. However, the universal quantification of the health burden associated with the UHI effect is complex. A study in the West Midlands, UK, assessed that the UHI contributed to almost half of the heat-related mortality [16]. On the other hand, people acclimatize to their climatic conditions. For instance, a study in London suggested relatively complete acclimatization to the UHI effect on heat–related mortality, but less clear evidence for cold–related mortality [17]. In Helsinki, Drebs [18] has previously measured and mapped the UHI over a period of one year and found its intensity to be of about 1 ◦C in the city centre on an annual level, with a situational variation up to about 4 ◦C. Furthermore, Kazmierczak [19] conducted a spatial analysis of social vulnerability to heat-related health impacts in Helsinki by using high-resolution spatial data on demographic and socioeconomic attributes of the population. The study inferred that exposure to high temperatures was high in many of the densely built areas of Helsinki. However, social vulnerability to heat, which is a combination of several indicators, was not especially high in most of the areas due to good adaptive capacity of the residents. Especially in urban areas, the interaction between air quality and the UHI effect needs to be considered in health impact assessments, including temperature-related mortality. As with the UHI, air quality varies notably geographically, depending on the characteristics and patterns of human activities and the built environment. The near-ground concen- trations of pollutant species also vary notably between urban and rural areas, e.g., [20]. According to a review study [21], temperature extremes modify the health effects of PM10 and O3 on both nonaccidental and cardiovascular mortality. A study on the Russian heatwave and wildfire in 2010 showed that interaction between high temperatures and air pollution from wildfires had an additive effect and contributed to more than 2000 excess deaths in Moscow [22]. However, it has been found in previous studies that most of the health-relevant air pollutant concentrations in the Helsinki region were lower than the corresponding values in major central and southern European cities, e.g., in [23,24]. The overarching aim of this study was to assess the temperature-related mortality in Helsinki and its surrounding region, with special emphasis on heatwaves. In this study, the day is defined as hot when the daily mean temperature exceeds 20 ◦C. This criterion is based on the health impact studies [2–4] and is used in issuing operational hot weather warnings in Finland. We also assessed the role of air quality in the temperature-related mortality in Helsinki. The specific objectives were: • To model the temperature–mortality relationships in the city of Helsinki and the wider Helsinki-Uusimaa hospital district (HUS) during the period 2000–2018, using distributed lag nonlinear modelling (DLNM). Atmosphere 2021, 12, 46 3 of 13 • To assess the deaths attributable to four intensive heatwaves, which occurred in 2003, 2010, 2014 and 2018, in Helsinki and its surroundings. • To evaluate the potential modifying effects of air quality on the temperature-mortality relationship in Helsinki. This study does not specifically address the influence of the UHI on heat-related mortality in Helsinki, but in the end, we also discuss its potential role in explaining the outcomes. 2. Materials and Methods The Helsinki-Uusimaa hospital district (HUS) in southern Finland geographically covers a land area of about 9097 km2. The capital of Finland, Helsinki, is located within the HUS hospital district by the coast of the Gulf of Finland (Figure1). The population in the study area has been growing about 10% per decade: during the study period, 2000–2018, the population in Helsinki increased approx. 18%, from 0.551 to 0.648 million and in HUS (including Helsinki) approx. 21%, from 1.376 to 1.667 million. The elderly population, 75 years or older, increased in Helsinki from 35,000 to 47,000 and in HUS from 69,000 to 117,000.