Increasing greenhouse gas emissions in the atmosphere is one of the most important problems at a global level [1] . Climate change and urbanization represent two of the most critical points that are inextricably linked [2] . Over the 20th century, the atmospheric concentration of greenhouse gases is increased due to human activities contributing to the global temperature increase [3] . Cities are responsible for 75% - 80% of global greenhouse gas emission [4] . It has been hypothesized that CO₂ emissions from road traffic worldwide will increase by 92% between 1990 and 2020 [5] affecting the environment, health and economy [6] . The problem is emphasized in those urban areas where vehicular traffic is the main source of greenhouse gases [7] and where the current urban dynamics is based on continuing trends of sprawl and as consequence with a high dependence on private motorized transportation can significantly contribute to the increase of gas emission rates [8] . Urban population is exposed to a greater risk of adverse health effects, which may be attributed to increased exposure to particulate matter and gaseous pollutants emitted by vehicular traffic. These pollutants pose a direct and serious hazard to living organisms in general and to humans in particular [9] . High levels of vehicle emissions in cities are related to the rising trend in allergic respiratory diseases [10] with a direct relationship between motor vehicle exhausts and acute or chronic respiratory symptoms in children living near high traffic level areas. Urban greening can reduce the population’s exposure to air pollution through the interception of airborne particles on the leaf surface or inside the leaf tissues via roots [11] or the uptake of carbon dioxide (CO₂) via stomata [12] . A growing body of research in outdoor environmental contexts, such as public urban spaces, residential settings, schools and hospitals shows that a view of the greening enhances health and reduces stress [13] . Moreover, research on urban parks demonstrates a positive relationship between availability of green areas and children and adolescents’ physical activity [14] as well as creative play [15] . Nevertheless, similar studies are not available for hospital healing gardens. Moreover, visitors and patients walking behavior varied in response to the hospital garden’s design characteristics (extrovert walking versus introvert walking) [16] . Dismissed as peripheral to medical treatments for much of the 20th century, gardens are back in style, now featured in the design of most new hospitals, according to the American Society of Landscape Architects. Previous studies suggest that design factors such as poor shading, planting and seating options influence usability of gardens in hospitals [17] and urban setting [18] . Several studies have found reduction in stress levels and health-related complaints among patients and staff [19] who were provided with windows overlooking gardens. Moreover, there has been indirect evidence that access to hospital gardens increases patient and staff satisfaction with the overall hospital experience [17] . Thus, healthcare specialists, architects and landscape designers should consider that the hospital environment might affect the mood, stress level and well-being of patients and their families [20] . In such context, the main objective of the present research was to assess the carbon dioxide (CO₂) concentration variations inside and outside the Policlinico Umberto I, one of the most important hospitals in Rome, considering the role of the greening in improving air quality. The collected data may be used for an inventory available for green furniture projects to improve the quality of life in protected areas such as hospitals, schools and shelters.

Urban greening can generate significant ecosystem services such as offsetting carbon emissions [23] [24] , removing air pollutants [11] reducing noise [25] , regulating microclimate [26] and favouring recreation and amenity [27] . Plants act as sink for CO₂ by fixing carbon during photosynthesis and storing the excess as biomass [28] . This plant function is particularly important in protected areas such as hospitals, schools and shelters [29] . Nevertheless, without detailed local data, urban vegetation managers cannot easily design and implement strategies to maximize a particular vegetation’s desired ecological functions, and thus, protect and enhance its value [30] .

Inside the Policlinico Umberto I, meadows cover 11,409 m² and the “group of trees” 18,773 m². In particular, at Ts1 most of the trees are evergreens while at Ts2 deciduous species. Among the evergreen trees, 45% are Quercus ilex, 20% Phoenix canariensis, 15% Pinus pinea, 15% Magnolia grandiflora and 5% Cupressus sempervirens. Among the deciduous trees, 60% are Tilia platyphyllos and 40% Platanus hybrida (mean value of Ts1 and Ts2, respectively). The considered trees have a height of 13.6 ± 6.8 m and a diameter of 0.50 ± 0.24 m, respectively (mean value of the tree species) (Table 1).

During the year, the highest CO₂ concentration is monitored in winter-spring (425 ± 8 ppm, mean value) (Figure 1) when the traffic density peaks (10 ± 8

car∙min⁻¹), decreasing by 17% in summer when the traffic density decreases by 50%, as confirmed by the significant positive correlation (r = 0.527, p < 0.05) between CO₂ concentration and traffic density. The day-time CO₂ trend (Figure 2) shows the highest concentration (433 ± 61 ppm, mean value) in the considered sites (i.e. Is and Os) in the first hours of the morning (9:00) decreasing by 12% from 11.00 to 13.00 in relationship with traffic density decrease (Figure 3). The 2.5% atmospheric CO₂ concentration increasing at 17.00 can be justified by the plant respiration/photosynthesis ratio increasing from the afternoon until the first hours of the morning. The results highlight the role of the greening in decreasing atmospheric CO₂ inside the Policlinico Umberto I. In particular, the “group of trees” decreases atmospheric CO₂ concentration by 20% than the outside, followed by meadows (14%) (Figure 4). Moreover, the greening has an

important role in improving people well-being particularly for hospitalized people. Studies show that post-surgical hospital patients with window views of green landscapes have a significantly shorter recovery period and reduced need for pain medication compared to patients with views of urban settings [31] and children’s health and emotional status benefit from day-care centres with vegetation [29] [32] . Moreover, planted trees can indirectly reduce CO₂ emissions associated with electric power production and consumption of natural gas by reducing the demand for heating and air conditioning in the buildings they shelter [33] . Trees represent also excellent regulators of air temperature by shade and transpiration [24] . Accordingly, our results show that outside the Policlinico air temperature was, on average, 17% higher compared to the inside (Figure 5), especially in March (30%) and in July (25%). An opposite trend was observed for air humidity, as attested by the correlation between the two variables (slope = −0.81; r = 0.45; p < 0.0001), resulting 18% higher inside than outside the hospital (Figure 5). Moreover, the evergreen species, such as Q. ilex, P. canariensis, M.

grandifolia, C. sempervirens and P. pinea, by their continuous photosynthetic activity throughout the year, contribute to reduce CO₂ also in winter when the traffic is the highest, as attested by the 67% higher traffic density compared to that monitored in spring and in summer. This role is very important in hospitals where the audience turnout is higher in winter. Several studies have shown the distribution of cumulative monthly deaths with a marked winter predominance [34] as well as an increase in respiratory and cardiovascular morbidity and mortality [35] .

On the whole, the results highlight that both evergreens and deciduous species contribute to decreasing air temperature in summer (T_a is 13% lower in Ts than in Es). Thus, plants in the city not only have an ornamental role but they may have also a role in regulating environmental functions. The type of greening (i.e. meadows and “group of trees”) and plant habitus (i.e. evergreens and deciduous species) are likely to affect this cooling potential and the carbon sink capability. Accordingly, our results show 8% lower CO₂ concentration in Ts compared to Ms attesting the greater carbon storage capability of trees. Green coverage can be incorporated in a geographic information system facilitating vegetation management through spatial analysis and identifying landscape features. Moreover, the choice of plant species might be set out favouring those species taking into account their own air amelioration capability. Plant traits of each species may be used to realise an inventory available for tree planting programs to ameliorate the quality of life.

The authors declare no conflicts of interest regarding the publication of this paper.

Gratani, L., Puglielli, G. and Catoni, R. (2019) Relationship between Atmospheric CO₂ Concentration and Vegetation in a Hospital Area: The Policlinico Umberto I in Rome. American Journal of Plant Sciences, 10, 1313-1324. https://doi.org/10.4236/ajps.2019.108095