How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’? An updated systematic review
This review updates a systematic review published in 2010 (http://www.environmentalevidence.org/completed-reviews/how-effective-is-greening-of-urban-areas-in-reducing-human-exposure-to-ground-level-ozone-concentrations-uv-exposure-and-the-urban-heat-island-effect) which addressed the question: How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’?
Searches of multiple databases and journals for relevant published articles and grey literature were conducted. Organisational websites were searched for unpublished articles. Eligibility criteria were applied at title, abstract and full text and included studies were critically appraised. Consistency checks of these processes were undertaken. Pre-defined data items were extracted from included studies. Quantitative synthesis was performed through meta-analysis and narrative synthesis was undertaken.
308 studies were included in this review. Studies were spread across all continents and climate zones except polar but were mainly concentrated in Europe and temperate regions. Most studies reported on the impact of urban greening on temperature with fewer studies reporting data on ground-level UV radiation, ozone concentrations (or precursors) or public health indicators. The findings of the original review were confirmed; urban green areas tended to be cooler than urban non-green areas. Air temperature under trees was on average 0.8 °C cooler but treed areas could be warmer at night. Cooling effect showed tree species variation. Tree canopy shading was a significant effect modifier associated with attenuation of solar radiation during the day. Urban forests were on average 1.6 °C cooler than comparator areas. Treed areas and parks and gardens were associated with improved human thermal comfort. Park or garden cooling effect was on average 0.8 °C and trees were a significant influence on this during the day. Park or garden cooling effect extended up to 1.25 kms beyond their boundaries. Grassy areas were cooler than non-green comparators, both during daytime and at night, by on average 0.6 °C. Green roofs and walls showed surface temperature cooling effect (2 and 1.8 °C on average respectively) which was influenced by substrate water content, plant density and cover. Ground-level concentrations of nitrogen oxides were on average lower by 1.0 standard deviation units in green areas, with tree species variation in removal of these pollutants and emission of biogenic volatile organic compounds (precursors of ozone). No clear impact of green areas on ground level ozone concentrations was identified.
Design of urban green areas may need to strike a balance between maximising tree canopy shading for day-time thermal comfort and enabling night-time cooling from open grassy areas. Choice of tree species needs to be guided by evapotranspiration potential, removal of nitrogen oxides and emission of biogenic volatile organic compounds. Choice of plant species and substrate composition for green roofs and walls needs to be tailored to local thermal comfort needs for optimal effect. Future research should, using robust study design, address identified evidence gaps and evaluate optimal design of urban green areas for specific circumstances, such as mitigating day or night-time urban heat island effect, availability of sustainable irrigation or optimal density and distribution of green areas. Future evidence synthesis should focus on optimal design of urban green areas for public health benefit.
Urban, Greening, Ground-level ozone, UV radiation, Heat-island, Updated systematic review
Climate change is likely to have direct and indirect impacts on human health. Changes in temperature, ground-level ozone (O3) and ultra-violet radiation (UV) are recognised public health issues, particularly in urban areas and their effects may be modulated by climate change. ‘Urban greening’ has been proposed as one possible intervention that may mitigate the human health consequences of these changes.
This review evaluates the available evidence on whether urban greening interventions, such as tree planting or the creation of parks, affects temperature, ground-level O3 and its precursors (volatile organic compounds, VOCs, or Nitrogen oxides, NOx) or UV within the surrounding urban area.
Searches were performed using electronic databases, internet search engines and specialist websites and articles subjected to pre-defined inclusion criteria in a series of filters (title, abstract and full text) to identify the subset relevant for the review. The relevant articles were then grouped according to the type of green site under study (e.g. park or green area, tree, green roof, ground vegetation) and the basic methodology used to collect data. Further information on methodology, study characteristics and results were recorded from each study. Due to the diversity of studies, a narrative synthesis was conducted for most of the literature but a meta- analysis was performed on a subset of studies when appropriate.
In total, 212 relevant articles were found. Most studies address the effects of greening on temperature, with O3 and UV being less studied.
The effects on temperature were assessed by three different methodologies: ground- level data collection, remote sensing data collection and modelling. The review focused on the 71 studies using ground-level data collection and these mostly suggest that a green site could be cooler than a non-green site. A meta-analysis conducted on park temperatures estimated that an urban park is on average around 1C cooler than a built-up site in the day. A number of variables were identified that could affect this relationship including factors of the green sites such as its area and vegetation type and other factors such as time of day or year. However, these studies were mostly site comparisons that sampled relatively small numbers of green sites. We did not find any studies that evaluated the effectiveness of an urban greening programme as part of a climate change adaptation strategy.
Studies on O3 and greening addressed a number of different questions. Empirical studies investigated the ability of plants to release volatile organic compounds (VOCs) and suggested that some plants may contribute to O3 production. Larger- scale empirical studies investigated the concentrations of ozone within urban green areas and demonstrated the complexity of interactions between O3, its precursors and temperature.
Few UV studies were identified and they mostly investigated the ability of trees to provide protection by reducing human exposure.
The review did not identify any studies that investigated the direct effects of urban greening on human exposure to high temperatures, O3 or UV, or any health-related consequences in the context of these variables. However, some articles were identified which predicted the human ‘thermal comfort’ of green and non-green environments, based on temperature and humidity measurements.
A considerable number of studies were identified that have aimed to assess how land cover including parks, green areas and trees affect temperature and to some extent O3. These studies suggest that it may be possible to use greening interventions as an adaptation strategy to climate change, however, the evidence is based on observational studies rather than more rigorous experimental examination. Most studies have investigated temperature differences between green and non-green sites within an urban area but the impact of greening on nearby non-green areas is a subject requiring more research. Studies on O3 indicate that any attempt to use greening to improve air quality would need to consider the biogenic emission of VOCs shown for some species, in order to estimate net air quality benefits. Few studies have been conducted on the effect of greening on UV.
There is insufficient evidence to guide the design of an urban greening programme. Further research is necessary to investigate the importance of the abundance and distribution of vegetation on the effectiveness of urban greening, for instance, the optimal distribution or parks; the difference between planting single versus clumps of trees and the importance of factors that may modify the significance of greening to temperature such as regional climate. Any urban greening programme that is implemented would need to be monitored to continue to test the hypothesis that they can improve urban areas for human health through reducing temperature, UV and ozone concentrations.
It is now widely accepted that climate change is occurring because of the accumulation of greenhouse gases in the atmosphere, which has arisen from the combustion of fossil fuels (IPCC, 2007). There has been growing interest in the potential mechanisms by which climate change might influence health (Confalonieri et al., 2007). In 2001, and updated in 2008, the UK Department of Health published a major report, ‘Health Effects of Climate Change in the UK’, which focused on the possible impacts of climate change on health (Department of Health and the Health Protection Agency, 2008). This report indicates that a significant impact of climate change on public health in the UK may occur through a number of main pathways including increased temperature, ground level ozone levels and ultra-violet radiation, which have a range of consequences for human health.
One predicted consequence of climate change is an increase in the intensity and frequency of heatwaves (RCEP, 2007). Heatwaves can present a serious health risk, for instance it has been predicted that there is a 1 in 40 chance that south-east England will have experienced a severe heatwave by 2012, resulting in 3000 immediate heat- related deaths (Dept of Health, 2008). Increased temperatures can be particularly problematic in urban areas, where temperatures already tend to be a few degrees warmer than the surrounding countryside; a phenomenon termed the ‘urban heat island effect’ (Rosenzweig et al. 2006). Concentrations of ground-level ozone are also predicted to increase, influenced by the effect of higher temperatures on ozone chemistry and the release of ozone precursors (RCEP, 2007). Ground level ozone levels can have considerable health impacts, in particular affecting respiratory diseases (Dept of Health, 2008). Increased exposure to UV radiation due to stratospheric ozone depletion and increased greenhouse gases also has a number of health consequences such as increased prevalence of skin cancer.
Strategies are needed for adaptation to the predicted effects of climate change on health. One strategy that has been proposed is to ‘green’ urban areas, essentially by increasing the abundance and cover of vegetation (Handley and Carter, 2006; Vandentorren et al., 2006; Gill et al., 2007). Vegetation, it is postulated, could counter some of the health consequences of climate change, in different ways. For instance, trees can provide shade, potentially reducing human exposure to high temperatures and UV radiation. Vegetation may reduce ozone levels by absorbing and trapping ozone precursors and pollutants (RCEP, 2007) and may allow adaptation to the urban heat island effect by increasing processes such as evotranspiration and reflection of radiation (Rosenzweig et al., 2006). This systematic review aims to consider the evidence on the effectiveness of ‘greening’ interventions in the urban environment in reducing urban temperature, UV and ground level ozone levels. The review will not consider the evidence underpinning the link between these environmental factors and health impact, which has already been extensively researched (Dept of Health, 2008).
The methodology of a systematic review is designed to ensure the review conclusions are objective and based on the best evidence available (Khan et al., 2003; CRD, 2004). Comprehensive searching, of both published and unpublished literature, and the application of specific inclusion criteria are used to retrieve relevant studies without bias. Articles included in the review are critically appraised and their findings summarised including a quantitative synthesis when appropriate. The findings of systematic reviews can guide the development of evidence-based policy and also highlight areas where current research is lacking.