Past and present summer heat conditions in Slovenia
The most recent climate classification of Slovenia divides the country into six regions based on 31 meteorological variables (Kozjek et al. 2017a): sub-Mediterranean climate region (Bilje), wet climate of hilly region, moderate climate of hilly region (Postojna), subcontinental climate region (Ljubljana, Celje, Murska Sobota, Novo mesto), subalpine climate region, and Alpine climate region. Summer maximum air temperatures do not reach extremely high values in Alpine and subalpine regions and in the wet climate of the hilly region. In agreement with ARSO (2017), averaged summer air temperature (TmeanJJA) has a great variability across Slovenia, from around 8 °C to more than 20 °C with maximum values in northeastern and southwestern part of Slovenia (Fig. 1a). The largest positive trend (trTmeanJJA) is clearly in the southeastern part, reaching over 0.5 °C per decade (Fig. 1c). The geographical distribution of the 30-year average of maximum air temperatures in July (TmaxJUL) is very similar to the one of mean summer temperature (Fig. 1b). Additionally, there are very high maximum temperatures in the southeastern part. The highest trend of maximum air temperatures in July (trTmaxJUL) is found in the northeastern and southeastern parts, around 0.6 °C per decade (Fig. 1d). Overall trends range from 0.4 to 0.6 °C per decade for both variables.
Also in agreement with ARSO (2017), at the six chosen locations, the TmeanJJA values ranged from 18.0 °C in Postojna to 21.4 °C in Bilje and TmaxJUL from 25.5 to 29.9 °C, respectively (see Online Resource 2). The highest trend of TmeanJJA was in Novo mesto (0.5 °C/decade) and the lowest in Bilje (0.4 °C/decade), while the highest trend for TmaxJUL was in Murska Sobota (0.5 °C/decade) and the lowest again in Bilje (0.4 °C/decade). Not surprisingly, the highest number of hot days was found in Bilje, reaching on average approximately 30.8 hot days per summer. The highest trends were in Murska Sobota (3.3 hot days/decade) and in Bilje (3.2 hot days/decade). The number of summer hot days at other stations in the subcontinental climate region ranged from 15.6 (Novo mesto) to 18.5 hot days/decade (Ljubljana). In Postojna, there were on average only 7.9 hot days/decade, with the lowest trend (1.7 days/decade).
As shown in Fig. 1 and Online Resource 2, mean summer temperatures and the number of hot days increased during 1961–2011, so their respective interaction was studied. The correlation r values between the measured and fitted data for the quadratic fit are 0.85 (0.75–0.91) in Bilje, 0.86 (0.77–0.92) in Postojna, 0.87 (0.79–0.93) in Celje, 0.89 (0.82–0.94) in Novo mesto, and 0.91 (0.84–0.95) in Murska Sobota. In Ljubljana, the linear fit provides a correlation coefficient of 0.86 (0.76–0.92). When summer mean temperature increases 1 °C, the number of hot days increases linearly, as reflected in the slope coefficient, which ranges from 5.1 (Postojna) to around 7 (Ljubljana, Celje, Novo mesto), 8 (Murska Sobota), or even 9.5 days/°C, or even worse (following the quadratic equation).
With the exception of the mountainous areas, values of heat stress (Fig. 2, first column) in all stations were between 22 and 26 °C for the summer mean (WBGTmean) and between 24 and 30 °C for the maximum index (WBGTx). Days with daily maximum heat stress above 27 °C rarely occur (on average) in present climate.
Climate change projections of temperature and heat stress
Climate change projections of temperature and heat stress indices were produced for the Slovene locations, obtaining the climate change signal as the difference between the projections for the period 2070–2099 with respect to 1981–2010. All indices are projected to increase in Slovenia by the end of the twenty-first century and the increments vary nonlinearly with the forcing scenario (Fig. 2). For instance, changes in summer mean (TmeanJJA) and July daily maximum (TmaxJUL) range from 1 °C for the lower emission scenario (RCP2.6) to 4.5 °C for the highest emission scenario (RCP8.5) in all Slovene stations. The number of hot days (HD) might increase 2–10 days per summer under RCP2.6 and up to 35 days under the highest emission scenario. HD changes present a larger spatial variability than the other temperature indices, and they are larger in the stations with the highest TmaxJUL in present climate (Fig. 1b).
Similarly to temperature extremes, summer mean and maximum heat stress are projected to increase from 1 to 3.5 °C depending on the emission scenario in all Slovenian stations (Fig. 2, fourth and fifth rows). The frequency of extreme heat stress (WBGTg27) will be accentuated in the locations where the frequency of HD largely increase, increasing up to 20 days in the stations in the center of the country and more than 30 days in Bilje under the strongest emission scenario.
Despite the model uncertainty in the climate change signal, there is overall good agreement in the mentioned changes (Fig. 3). Model uncertainty is quite similar across all stations for summer mean temperature and heat stress. In Bilje, the uncertainty in the number of days with extreme heat stress (WBGTg27) is especially large, ranging from 10 to 50 days for RCP8.5. It is interesting to see that, even under the low (RCP2.6) and moderate (RCP4.5) emission scenarios, important increases may occur in the number of hot days and high heat stress risk due to the warmer and humid conditions in the sub-Mediterranean climate region.
Case study: the odelo factory
The first approach for an analysis of the temperature conditions in a workplace should include general information on the local temperatures during the year. For this purpose, we analyzed the data at Celje station in the period 1981–2015 and compared it with the year 2016, during which we conducted the study. Monthly values of WBGT had higher variability in winter and higher maximum values in July (Fig. 4, top left), always under 30 °C (median under 25 °C). The variability is comparable among summer months, with an interquartile range of around 4 °C. Values can, however, differ substantially from year to year. For example, in the year 2016 (Fig. 4, top right), interquartile ranges are more diverse in summer with a median of 24.2 °C in July, which is higher than the measured average in 1961–2015. Summer of 2016 was not detected as very hot, with only one heat wave in Slovenia, but still within the range of the natural variability. Thus, it can be considered representative of summer conditions in Celje.
The odelo d.o.o. company installed a ventilation system, but it cannot dissipate the excessive heat produced by the injection molding process. In July 2016, the correlation coefficient of the indoor temperatures (measured at both heights) with the temperature outside odelo was 0.85, with indoor air temperatures being higher, and with lower variability than outdoor air temperatures (Fig. 4, lower panel). As an example, even with outdoor air temperature around 15 °C, the indoor temperatures measured at 1.5 m height were between 25 and 30 °C. Relative humidity was significantly lower inside the plant compared to the outside relative humidity.
Since WBGT depends on air temperature and relative humidity, a different interaction can be observed between air temperature and WBGT at the workplace and at Celje station (Fig. 4, lower panel). At the workplace, the pattern of daily WBGT data (inside the factory) follows the pattern of the external air temperature data, with WBGT systematically around 6 °C lower. At the meteorological station (with outside air temperature and relative humidity significantly higher than inside the factory), the diurnal cycle of temperature data stretches with respect to the WBGT counterpart. Minimum daily values did not differ much, but the maximum external air temperature values were higher by about 2.4 °C. In the first half of July 2016, WBGT values at the injection molding workplace were between 20 and 25 °C. During the latter half of July, WBGT increased progressively above 25 °C, attaining maximum values of 28.3 °C.
Concomitant with the analysis of the conditions in the factory during the summer of 2016, we also surveyed the workers regarding their perception of the temperature at the workplace during heat waves. Temperature conditions were suitable for less than 4% of those completing the survey (Fig. 5, top left). For the majority, it was warm, hot, or very hot. There was a statistically significant difference (p < 0.001) in the number of women compared to men that perceived the working conditions as “very hot,” suggesting that they had a higher sensitivity to the hot conditions.
Working clothes were very comfortable for less than 10% of employees, with the majority of workers reporting clothing comfort between comfortable and uncomfortable (Fig. 5, top right). There was no significant difference between the males and females. For around 20% of the workers, the clothes were not comfortable at all.
The age of the workers was homogenously distributed and there was also a large group of younger workers. This needs to be considered when analyzing the questionnaires. Namely, the younger workers do not have as much experience of heat waves as the older workers. Therefore, their answers may only apply to a shorter and recent time period. Almost twice as many men than women replied that they have not been increasingly more exposed to heat stress (Fig. 5, middle left), and more than twice as many women than men replied that climate change is the main reason for experiencing heat stress (p < 0.001). The prevalence of women in noticing the change could be related to their higher sensitivity to heat stress conditions as shown before (Fig. 5, top left).
More than 50% of the workers reported having better climate conditions at home and on their way to work (Fig. 5, middle right), with non-statistically significant differences between men and women. However, 35% of the workers reported that their situation regarding perceived temperature was worse on the way to work than at work, with a greater percentage in women than men.
All acknowledged that heat stress can cause heat strain symptoms (Fig. 5, bottom left) leading to heat-induced illness if the problem is not resolved (Fig. 5, bottom right), which may ultimately have a fatal consequence. Since becoming operational in 2005, there has only been one incident of heat stroke in odelo and 13 incidences of heat-induced health problems that required hospitalization. Thirst and excessive sweating are the first signs of hot ambient conditions, reported by men (> 70%) and women (> 80%) in the factory (differences between males and females was not significant). Tiredness (p < 0.001), confusion (p < 0.001), and dizziness (p < 0.05) are more commonly perceived by women (81, 19, and 39%, respectively) than men (56, 12, and 9%, respectively). Enhanced stress due to heat is experienced by 28% of men and 29% of women.
Gender differences are also evident among the reported heat-induced health problems (Fig. 5, bottom right); 39% of the male workers did not report any health problems, whereas 37% were affected by a headache and 47% by exhaustion. These percentages were much higher for female workers, with 73% (p < 0.001) and 64% (p < 0.01), respectively. Furthermore, 33% of the women have experienced nausea or vomiting (p < 0.001) and 16% prickly heat (p < 0.01), while only 6% of the male group reported the occurrence of these symptoms. There were also cases of muscle cramps and fainting in both gender groups and in the female group also cases of heat cramps and heat stroke.