|Home | About | Journals | Submit | Contact Us | Français|
In recent years the developing world, much of which is located in the tropical countries, has seen dramatic growth of its urban population associated with serious degradation of environmental quality. Climate change is producing major impacts including increasing temperatures in these countries that are considered to be most vulnerable to the impact of climate change due to inadequate public health infrastructure and low income status. However, relevant information and data for informed decision making on human health and comfort are lacking in these countries.
The aim of this paper is to study and compare heat stress conditions in an urban (city centre) and rural (airport) environments in Akure, a medium-sized tropical city in south-western Nigeria during the dry harmattan season (January–March) of 2009.
We analysed heat stress conditions in terms of the mean hourly values of the thermohygrometric index (THI), defined by simultaneous in situ air temperature and relative humidity measurements at both sites.
The urban heat island (UHI) exists in Akure as the city centre is warmer than the rural airport throughout the day. However, the maximum UHI intensity occurs at night between 1900 and 2200 hours local time. Hot conditions were predominant at both sites, comfortable conditions were only experienced in the morning and evenings of January at both sites, but the rural area has more pleasant morning and evenings and less of very hot and torrid conditions. January has the lowest frequency of hot and torrid conditions at both sites, while March and February has the highest at the city centre and the airport, respectively. The higher frequencies of high temperatures in the city centre suggest a significant heat stress and health risk in this hot humid environment of Akure.
More research is needed to achieve better understanding of the seasonal variation of indoor and outdoor heat stress and factors interacting with it in order to improve the health, safety, and productivity of Akure city dwellers.
The UN predicts that 60% of the world's population (~5 billion people) will be living in cities by the year 2030 and that nearly all the population growth will be in the cities of developing countries. This rapid population growth will largely drive the extent and rate of global environmental changes and many of these changes are related to the climate and atmospheric composition of cities, including the canopy layer urban heat island (UHI) – the observed warmth of the urban core compared to its rural surroundings, heat stress, and various forms of air pollution. Climate change will impact living and working environments negatively and create health threats for millions of people (1, 2). The average global temperature is increasing and it is estimated that it will go up a further 1.8–4.0°C (estimated average 3.0°C) by the year 2100 (1), depending on actions to reduce greenhouse gas emissions. The extent of local climate change will vary depending on geographic and local meteorological conditions. Increasing local ambient temperature implies higher human exposure to heat that, during hot seasons in hot regions of the world, can create very severe heat stress and health risks for people who are not able to afford neither the cost of air conditioning and other cooling methods nor the cost of energy required to run them. Both general living environments and working environments are affected. The latter may create impacts both on workers' health, productivity, and socio-economic development (3–5). Workers in low- and middle-income tropical countries are likely to be at the highest risk of excessive heat exposure. The aim of this paper is to compare heat stress conditions in an urban (city centre) and rural (airport) environments in Akure (7.25°N, 5.20°E), a medium-sized tropical city in south-western Nigeria during the dry harmattan season (January–March) of 2009.
We analysed heat stress conditions in terms of maximum, minimum, and mean hourly values of the thermohygrometric index (THI), defined by air temperature and relative humidity (6) using the equation below:
Where air temperature (T) is measured in degrees Celsius, with RH as the relative humidity in a percentage. This index has been used in many bioclimatic investigations (7–9). The THI categories used here are as listed in Table 1.
Recently widespread discrepancies in the classification of so-called urban and rural measurement sites for defining the UHI magnitude have been reported (10), and highlight the need for a universally applicable landscape classification scheme for defining and measuring the UHI magnitude.
For clarity about the detailed character of the reference sites used in this study, each site is classified according to the new local climate zone classification (LCZ) scheme (11) that corresponds to the Urban Climate Zone (UCZ) scheme (12).
Simultaneous measurements of T and RH commenced in December 2008, using a shielded portable Lascar EL-USB-2 temperature/humidity data loggers, sampled at 5 min intervals, and mounted on a lamp post above head height (3 m) in the city centre, classified as ‘compact low rise’ local climate zone and at the same height on a mast at the airport, classified as ‘sparsely built’ local climate zone (11), which is located about 15 km east on the outskirts of the city. See Fig. 1 for the location map of the sites, labelled 1 and 2 for urban and rural, respectively, and insert for the site photographs and sky view. Using this scheme, the estimate of the magnitude of the UHI that is based on air temperature alone becomes UHI = T Urban–T Rural.
Descriptive statistics were applied with one-way analysis of variance (ANOVA) to detect significant differences (95% confidence interval) between the mean values of THI at the urban and rural site.
Fig. 2 (a–c) shows that the UHI exists in Akure throughout the day in January to March. The highest diurnal UHI intensity is also observed in January. The figure further shows that the maximum UHI intensity occurs at night between 1900 and 2200 hours local time. This is the first complete diurnal data reported for Akure. Earlier data reported for Akure have been restricted to the daytime period (13). This result, therefore, provides new information on the diurnal characteristics of the UHI in Akure.
Table 2 reveals that the urban city centre was far less comfortable than the rural airport and five THI categories (cool, comfortable, hot, very hot, and torrid) can be identified. Hot conditions were predominant at both sites, comfortable conditions were only experienced in the morning and evenings of January at both sites, but the rural area has more pleasant morning and evenings and less of very hot and torrid conditions. The city centre did not register any cool conditions at all with very hot and torrid afternoons. The month of January recorded the lowest frequency of very hot and torrid conditions, while March recorded the highest. Comfortable conditions were restricted to the morning in January and brief periods in the evenings of all months for minimum THI. The rural area registered cool conditions in the morning in January. Though the mean THI values and frequencies of ‘very hot’ and ‘torrid’ conditions for the urban site were consistently higher than that at the rural site (Table 2), one-way ANOVA also indicated that there was no significant difference (p<0.05) between the mean, maximum, and minimum values of THI between the urban and rural site for the months of February and March. However, there exists significant difference (p<0.05) between the mean and minimum values of THI between the urban and rural site for January (Table 3).
Unlike temperate regions where higher warmth of the city is beneficial in winter, it is a significant thermal heat stress risk in hot humid environments like Akure.
The characteristics of the UHI and heat stress in Akure have been investigated and results reveal some interesting new findings on the diurnal characteristics of the urban–rural temperature and THI differences in the city. The UHI has been found to occur throughout the day and night between January and March. Results also show that the highest UHI intensity during the study period occurs at night between 1900 and 2200 hours in January. This result is just coming to light now as earlier studies were restricted to the daytime. ‘Hot’ conditions were predominant at both sites, comfortable conditions were only experienced in the morning and evenings of January at both sites, but the rural area has more pleasant morning and evenings and less of very hot and torrid conditions. The month of January recorded the lowest in hotness and torridness while March recorded the highest at both sites. The higher frequencies of high temperatures in the city centre suggest a significant heat stress and health risk in this hot humid environment of Akure.
The impact of elevated heat stress on the human work capacity and health in work situations is an effect of global climate change that little attention has been paid. The potential health risks and reduced worker productivity due to climate change are substantial. These will hamper socio-economic development in affected countries unless appropriate preventive measures are taken in the planning processes for workplaces and urban development. The results of the one-way ANOVA that indicate no significant difference (p<0.05) between the mean, maximum, and minimum values of THI between the urban and rural site for February and March suggest that the same energy demand will be required for cooling at both the urban and rural sites in Akure and this has an implication for thermal comfort planning and decision making in the city. While the current study has laid the foundation for the acquisition of necessary data for the planning and decision-making processes, more research is needed to achieve better understanding of the seasonal variation of indoor and outdoor heat stress and factors interacting with it in order to improve the health, safety, and productivity of Akure city dwellers. The lack of research facilities and funding are major limitations to urban climate research in Nigeria. Availability of measurement and data logging systems for the four environmental factors that human thermal comfort depend (airflow [wind], air temperature, air humidity, and solar radiation) will greatly enhance further research.
This work was supported by Twins & Associates Ltd, UK. Publication support from Dr. Tord Kjellstrom is also gratefully acknowledged.