Designing healthy edible cities: investigating the environmental and spatial factors affecting urban fruit safety

ABSTRACT

This study aims to investigate the factors affecting lead and cadmium levels in urban fruits, a topic that has received little attention despite potential health risks. Tehran’s Tarasht neighborhood was chosen due to heavy traffic and the presence of numerous urban gardens with fruit trees. Specifically, this study explored how barriers (defined as artificial structures like walls), urban fruit tree height, distance from the sampling site to the street, and fruit type influence the content of lead and cadmium in these urban fruits. The results revealed that sites taller than 10 meters exhibited the highest cadmium levels, while sites without barriers or with 4-meter barriers demonstrated the highest lead content. Barrier width also had a significant influence, with unobstructed sites showing the highest lead content, and sites with 41-50-meter width barriers displaying the highest cadmium levels. Urban fruit tree height affected lead content, while cadmium levels remained unaffected. Additionally, heavy metal absorption was influenced by the distance to the street and the type of fruit. These findings can be used in the future for the development of guidelines for healthy and edible green infrastructure to reduce pollutants and promote safer urban horticultural practices.

KEYWORDS:

• Edible street trees
• traffic density
• edible green infrastructure
• cadmium
• urban agriculture

Introduction

Edible green infrastructure has piqued the interest of experts and city managers worldwide due to its numerous benefits, which include nutrition, social welfare, and increased urban resilience (Russo et al. 2017). Concurrently, edible urban projects encounter a crucial challenge concerning the assessment of the health index of agricultural goods and fostering public trust in their consumption (Antisari et al. 2015). Researchers have recognized the potential health risks linked to the unintended intake of soil contaminants or the consumption of vegetables and fruits cultivated in polluted urban settings, particularly concerning vulnerable populations, such as children and adults (Audate et al. 2019). Despite increasing interest in edible green infrastructure and edible cities, the formulation of universal guidelines for fruit production in urban areas remains limited, alongside a dearth of comprehensive knowledge on effective design and management practices (Gori et al. 2019).

The safety of urban food, particularly concerning perennial woody species such as shrubs and trees that endure prolonged exposure to high pollution levels over extended periods, represents a highly pertinent concern (Romanova and Lovell 2021). In urban areas, heavy metals (e.g. lead, mercury) can result in a wide range of detrimental health effects when accumulated (Cooper et al. 2020). Studying their presence in woody edible plants (e.g. apples, apricots) is crucial due to potential health risks and diverse contamination sources.

However, the literature presents conflicting findings, necessitating the identification of factors influencing heavy metal contamination in woody edible species. For instance, Sevik et al. (2020) conducted a study on cherry, plum, mulberry, and apple trees in various traffic zones in Turkey, revealing increased concentrations of nickel (Ni) and cobalt (Co) in different plant parts depending on traffic density, with fruits exhibiting potentially elevated heavy metal levels. Conversely, Cooper et al. (2020) found that edible fruit tissues in an urban community garden exhibited lower heavy metal and arsenic accumulation compared to leaf tissues in lettuce and Swiss chard. While studies like Romanova and Lovell (2021) point to the understudied nature of contamination in edible woody species (e.g. fruit trees), research on it remains limited. This highlights the need for further exploration of spatial patterns in urban design and landscape architecture promoting healthier urban agriculture. Addressing this research gap, our study investigates whether the spatial characteristics of planting sites, such as proximity to roads, along with environmental factors like traffic burden, influence the metal content of edible woody species in Tehran. Additionally, we measure and compare the heavy metal content of urban fruits with food health standards to assess potential health risks associated with consuming these fruits.

Materials and methods

Tarasht Neighbourhood is one of Tehran’s longstanding communities situated within its second region. Located on the western and southwestern periphery of the city, Tarasht boasts a rich history.

The geographical extent of Tarasht stretches northward to Sheikh Fazlullah Nouri highway, southward to Azadi Street bordering District 9 of the Tehran Municipality, westward to Mohammad Ali Jinnah Highway bordering District 5, and eastward to Shahid Timuri and Shahid Javad Akbari Blvd. Information gleaned from the 2015 population and housing census indicates that Tarasht is home to roughly 51,751 residents and includes an area of approximately 130 hectares (Statistical Center of Iran 2017).

Historically, due to the abundance of gardens and agricultural lands, a significant portion of Tarasht’s residents were employed in horticulture and agriculture. However, as the neighborhood’s density has increased and the area of gardens and agricultural fields has shrunk, the residents’ occupations have correspondingly transformed.

In fact, Tarasht holds the distinction of being one of Tehran’s oldest western villages, progressively incorporated into the urban fabric over time. Despite this integration, the neighborhood retains a reputation for its temperate climate and lingering traces of fruit orchards, preserving a sense of its past while offering a green and pleasant environment.

The rationale behind selecting this area for the present study stems from its proximity to Tehran’s strategically located traffic arteries on all four sides. Furthermore, the presence of numerous private, multi-tiered, fruitful urban gardens within the neighborhood presents a compelling subject for investigation.

Tarasht’s gardens have long been celebrated as some of Tehran’s most renowned orchards, brimming with a diversity of fruit trees. Even today, pockets of these gardens and orchards thrive within the neighborhood. A total of 103 fruit samples were randomly collected from 14 locations (Figure 1), including gardens, houses with courtyards, streets, and alleys, during the fall of 2022.

Figure 1. Left: Sampling of the 14 locations in Tarasht (Image Source: Google). Right: Harvested fruits.

We used specific definitions for the terms ‘gardens’ and ‘houses’. Gardens were defined as private, enclosed spaces exceeding 500 square meters, characterized by a predominance of productive vegetation. Conversely, houses were categorized as real estate units under 500 square meters, where at least 60% of the area is dedicated to buildings, with the remaining space functioning as a multi-purpose yard. The selected fruits included persimmons, olives, apples, oranges, walnuts, blackberries, pomegranates, grapes, and figs. Heavy metal content in the fruit samples (specifically lead and cadmium) was determined using atomic absorption spectroscopy. These two metals are of particular concern in Iran due to their widespread presence in the environment and their potential health risks (Ghoochani et al. 2018, Zamani et al. 2020). Given the prevalence of these metals and their potential impact on edible green infrastructure, we considered several factors that may influence heavy metal accumulation, including Distance to the nearest street, Distance to the nearest main arterial road, Height (m) of barrier (bh), Width (m) of barrier (bw), Degree of enclosure by barriers (de), and Overall traffic burden. To account for traffic volume, we extracted Overall Traffic Burden (OTB) data from Google Maps. Normal traffic data, representing average conditions for a chosen day of the week and time, was used to avoid bias from temporary events. Google Maps uses a color-coded system (green, orange, red) to illustrate traffic flow, which we converted into a categorical OTB variable (L/M/H). By matching sampling locations in the Tarasht Neighborhood with corresponding traffic data on Google Maps, we assigned OTB values based on the displayed color code.

The collected data underwent rigorous descriptive and inferential statistical analyses using SPSS Statistics. The analyses encompassed conducting an Analysis of Variance (ANOVA) to assess group differences, and post hoc comparisons were performed using the Least Significant Difference (LSD) test to identify specific pairwise differences between groups. These analyses aimed to identify spatial patterns of heavy metal accumulation and assess the health risks associated with consuming urban fruits in different environments.

Results and discussion

The study found low levels of cadmium and lead in urban fruits in Tehran, which meet national standards but are significantly below them. The distance from the street to the sample site strongly influences cadmium absorption in fruits, and site-specific factors such as traffic, barriers, and sampling height also play a role (Table 1). Apples and stone fruits accumulate more lead than nuts, consistent with previous research (Säumel et al. 2012, von Hoffen and Säumel 2014). Grapes had the highest cadmium levels (32.0 ppb), while olives had the lowest (24.0 ppb). Bitter oranges had the highest lead levels (41.16 ppb), while grapes (78.3 ppb) and olives (79.3 ppb) had the lowest.

Table 1. The table presents the results of a one-way ANOVA analysis for various factors and their interactions.

FACTOR	Cd		Pb
	F	p	F	p
Site piking	2.306	0.107	0.719	0.491
Fruit type	2.829	0.066	0.946	0.393
Overall traffic burden	0.374	0.689	2.598	0.081
Distance to nearest road	2.334	0.050*	1.097	0.37
Distance to the nearest main arterial road	1.089	0.359	0.743	0.53
Presence/absence of barrier	1.429	0.241	1.582	0.201
Height (m) of barrier (bh)	4.052	0.010**	0.97	0.412
Width (m) of barrier (bw)	2.688	0.028*	0.361	0.873
Degree of enclosure by barriers (de)	1.266	0.288	1.992	0.144
Sample height	0.21	0.889	2.911	0.04
Species Sample height	1.37	0.172	4.853	0.000**
Species Overall traffic burden	1.024	0.443	2.503	0.007**
Species barrier	1.008	0.466	2.126	0.017*
Species Height (m) of barrier	2.605	0.004**	2.627	0.003**
Species Width (m) of barrier	0.985	0.482	3.22	0.001**

p = p-values F = F-values The p-values indicate the significance level of the tests, and F-values represent the test statistics. An alpha level of 0.05 was used for all tests. *indicates significance at the 0.05 level, **indicates significance at the 0.01 level.

This occurrence can be attributed to the protective properties of nut fruit shells, such as walnuts, which act as a barrier, separating the edible portions from the surrounding environment during growth, thus mitigating the impact of air pollution (Wyttenbach and Tobler 1998, Rodushkin et al. 2008). In contrast, the findings of this investigation contradict those of a study conducted in Copenhagen, Denmark, where nuts demonstrated elevated lead levels when compared to berries and stone fruits (Samsøe-Petersen et al. 2002).

Furthermore, some researchers propose that nuts with robust protective shells accumulate minimal lead or cadmium (von Hoffen and Säumel 2014). However, this assertion contrasts with our findings. Despite the walnuts harvested from Tarasht orchards showing one of the lowest accumulations of lead and cadmium, their toxic content was still significant and cannot be overlooked. Polluted sites in urban environments can increase the contents of trace metals in fruits grown under low traffic load (Hashisho and El-Fadel 2004, Gori et al. 2019, Demirhan Aydın and Pakyürek 2020). However, the trace metal content of urban fruits and vegetables does not always correlate with different degrees of soil pollution (Samsøe-Petersen et al. 2002). Absorption of lead from the soil is generally passive and lead is not transferred directly to the edible parts of the fruit tree (Ward and Savage 1994). Deposition of trace metals from the atmosphere onto fruit surfaces seems to have a stronger effect than absorption from the soil (Samsøe-Petersen et al. 2002, Madejon et al. 2006). As a result, lead from traffic is more likely to be present on rough surfaces such as bark, which is uneven compared to the relatively smooth fruit surface bark (Ademoroti 1986). Consequently, unwashed olive fruits grown in damaged soils did not show significant differences in Cd and Pb content compared to fruits grown in healthy soils (Madejon et al. 2006).

Traffic-derived pollutants exhibit a significant escalation along streets in contrast to non-street locations. The pollutants deposited in the soil demonstrate prolonged persistence, serving as a continual source of pollution in urban settings (Querol et al. 2007, Hjortenkrans et al. 2008).

In this study, the presence of cadmium in the samples could be attributed to locales characterized by elevated air pollution, leading to an increased heavy metal load, especially cadmium, in the soil. Additionally, diverse rainfall patterns, including the rise of acid rain in Iran over the last few decades (Jalali and Peikam 2022), may enhance the mobility of cadmium as a soil pollutant. The utilization of chemical fertilizers, pesticides, sewage sludge, and the deposition of atmospheric pollutants can further augment the concentration of cadmium in the soil. Consequently, the direct correlation of cadmium with traffic load and proximity to the street may be less significant, as other sources such as toxins, chemical fertilizers, and pesticides may contribute to cadmium accumulation.

In addition, the height of the sampling site has a significant effect on the lead content. According to the results of the LSD test, there was a significant difference in the average lead levels between samples obtained from the ‘Low’ traffic condition and those from the ‘Medium’ traffic condition (Mean Difference = −3.75, Std. Error = 1.70, p = 0.031). Similarly, a significant difference was observed between samples from the ‘Low’ traffic condition and the ‘High’ traffic condition (Mean Difference = −0.06, Std. Error = 1.37, p = 0.965). Additionally, the ‘Medium’ traffic condition showed a significant difference in lead levels compared to the ‘High’ traffic condition (Mean Difference = 3.75, Std. Error = 1.70, p = 0.031). Regarding cadmium, the LSD statistical test showed that traffic did not affect the amount of cadmium in the samples as the p-values for all comparisons exceeded the significance level of 0.05.

In this research, it was observed that the highest amount of lead is observed in street fruits and the lowest amount of lead is in garden fruits. Also, the highest amount of cadmium is observed in garden fruits and the lowest amount is in street fruits.

In the current study, the reason for the increased concentration of cadmium in plants with a greater distance from the street compared to plants with a closer distance to the street can be seen in the residents’ greater use of agricultural fertilizers and also the type of irrigation water. So that the infiltration of urban (municipal) irrigation water is more in the soil near the street, and generally this water can be from purified industrial effluents, the concentration of heavy elements of which is standardized, and the absorption of heavy metals by the deep roots of non-fruitful street perennial plants is more, so it causes reducing the concentration of heavy metals in the edible plants. On the other hand, the consumption of phosphorus fertilizers by garden residents is higher than municipal consumption, which will increase the concentration of cadmium in the fruits produced in gardens. The concentration of heavy elements such as lead and cadmium can be related to the age of the plant as well as lack of horticultural c.

In addition, our study showed that barriers (height and width of the barrier) between the planting site and the street reduce lead in edible fruits of fruit trees. So, with increasing the height and width of the barrier, the amount of lead in the fruits decreased. However, such an effect was not proven in the case of cadmium. Since particles washed from houses and painted walls double the lead content in surface soil (Awino et al. 2022), the proximity of buildings and walls can also increase the content of trace metals in surface soil. The data of the present research showed that increasing the height and width of a barrier reduces the content of lead in the edible part of fruits. Traffic-related particles can be immobilized on the surfaces of vegetation barriers such as hedgerows, thereby reducing the deposition of traffic-related pollution on fruits (Baldauf 2017, Przybysz et al. 2020).

Healthy edible green infrastructure: landscape design and spatial considerations for mitigating heavy metal contamination in urban fruits

The study’s key findings highlight the crucial role of landscape design and spatial considerations in developing sustainable and healthy edible green infrastructure. Factors such as the presence and height of barriers around sites significantly influenced heavy metal content in fruits (Table 2). Barrier heights above 10 meters were associated with higher cadmium levels, while sites without barriers or with 4-meter barriers had elevated lead content. However, sites with 8 and 10-meter barriers exhibited reduced lead content. Furthermore, the width of barriers and fruit tree height played significant roles in heavy metal accumulation. The distance from the sampling site to the street impacted cadmium absorption in fruits, and the height of the sampling site affected lead content.

Table 2. The amount of lead and cadmium in the samples based on the height and width of the barrier.

	Content of trace metals [µg/kg DW] based on the height of the barrier
	Cd		Pb
height of the barrier	N	Mean	N	Mean
Up to 4 (m)	17	0.25	17	7.89
4-8 (m)	34	0.26	34	5.55
8-10 (m)	16	0.28	16	7.2
Without barrier	8	0.25	8	7.74
	Content of trace metals [µg/kg DW] based on the Width of the barrier
	Cd		Pb
Width of the barrier (m)	N	Mean	N	Mean
Without barrier	8	0.25	8	7.74
10–20	10	0.25	10	7.23
21–30	24	0.26	24	6.49
31–40	12	0.25	12	4.97
41–50	16	0.28	16	7.20
> 50	5	0.25	5	7.05

The width of the barriers also played a role in heavy metal accumulation, with the highest lead content observed in fruits harvested from unobstructed sites. Fruits from sites with 41-50 meter width barriers displayed the highest cadmium levels, potentially due to penetration from water and soil sources.

Fruit tree height affected lead content, with the highest levels found in fruits harvested at heights of 2.5-1.91 meters, likely due to proximity to polluted air. However, no significant effect of height on cadmium content was observed.

Moreover, the distance from the sampling site to the street significantly influenced cadmium absorption in fruits, and the height of the sampling site had a significant effect on lead content. The type of fruit and sampling height also simultaneously influenced the amount of lead absorption.

A comparison of fruits from different types of sites (garden, street, house) revealed that street fruits had the highest lead levels, while garden fruits displayed the highest cadmium content (Table 3).

Table 3. Comparison of Pb and Cd content in different harvesting sites (house, garden, street).

	Harvesting sites	Mean	Median
Pb	Home	7.611	4.789
	Garden	6.116	4.988
	Street	7.741	8.035
Cd	Home	0.254	0.248
	Garden	0.264	0.258
	Street	0.248	0.246

The study’s findings underline the importance of various factors in creating healthy edible green infrastructure. Incorporating barriers, maintaining minimum distances between fruit trees and streets, and utilizing plant hedges or building walls can minimize lead pollution, particularly in high-traffic areas. Designing parks and urban green spaces with specific tree types in designated areas can further reduce the impact of lead on edible crops.

For cadmium, factors unrelated to local traffic, such as tree maintenance, water and soil quality, and gardening practices, should be emphasized among Tarasht Neighborhood residents to mitigate its effects on fruit health. Figure 2 graphically summarizes the results of this research, which can be used by landscape architects and urban planners.

Figure 2. Impact of barriers and tree height on lead and cadmium levels in urban fruits. Gardens are private, enclosed spaces exceeding 500 m², primarily covered by productive greenery. In contrast, houses are real estate under 500 m², with at least 60% dedicated to buildings and the remaining area as a mixed-use yard.

However, the data from this study is only valid within the specific geographic areas investigated. Since pollutant contamination on edible fruits is site-specific, further studies are needed to develop generalizable design guidelines for healthy edible green infrastructure. The spatial aspects of this study can still improve GIS-based research. For example, Kazemi and Hosseinpour (2022) used Analytical Hierarchy Process (AHP) and GIS techniques to identify the most suitable sites for developing urban agriculture in Mashhad, Iran. Our findings can be used, for instance, to create buffer zones in GIS studies between pollution sources and potential areas for edible green infrastructure.

This research was conducted in Tehran, where authorities often hesitate to promote planting edible fruit trees due to concerns about potential health risks associated with heavy metal accumulation in the fruits. This study contributes valuable insights by demonstrating that, under certain conditions, fruit trees in Tehran may not pose a significant threat to human health. The findings highlight the influence of factors like barriers, tree height, distance from traffic, and fruit type on heavy metal content. However, it is crucial to acknowledge the limitations of this study. Further research across diverse urban contexts with varying pollution levels is necessary to develop more detailed and quantitative design guidelines for safe and sustainable urban fruit tree planting practices. Policymakers need evidence-based approaches to implement policies promoting edible green infrastructure development in edible public gardens (Amani-Beni et al. 2022).

According to Kazemi and Hosseinpour (2022), research methodologies and supporting data are crucial for identifying safer urban areas for agricultural development, raising public awareness about the concept, and promoting the use of urban agriculture’s products.

Conclusion

This study is the pioneering research on edible green infrastructure in Iran, and its findings can serve as a foundation for developing sustainable guidelines in designing and planning future edible green infrastructure projects.

Based on the research findings, the cadmium and lead content in fruits harvested from the fruit trees in Tarasht Neighborhood falls below the maximum tolerance specified in Iran’s national standard, rendering them safe and suitable for consumption. An essential outcome of this study is the identification of key factors contributing to healthy conditions in edible green infrastructure design and planning. Ensuring minimal distance between fruit tree planting spaces and high-traffic streets, incorporating plant, and building barriers to shield planting beds from traffic arteries, and adopting health-oriented planting designs in gardens and agricultural green spaces can all lead to healthier fruits.

Furthermore, it is recommended to prioritize the planting of significantly safer edible urban fruit trees in areas surrounded by buildings and abundant vegetation. This strategy effectively mitigates the impact of traffic-induced air pollution on these fruits, resulting in a safer food source.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This research was conducted with the financial support of the Tehran Urban Research and Planning Center (Tehran Municipality). The work was supported under contract number 137/739043.

Notes on contributors

Mohammad Reza Khalilnezhad

Mohammad Reza Khalilnezhad is an academic staff at the University of Birjand’s Faculty of Arts. Specializing in landscape architecture and environmental design, he holds a Ph.D. from Technische Universität Kaiserslautern, Germany, obtained in 2016. His research focuses on urban agriculture design, productive landscapes, and Persian gardens. He is a proponent of original research in landscape architecture, particularly Persian Garden studies. With a background that includes a BSc from the University of Tabriz and an MSc from the University of Tehran, he is a respected academic in his field.

Mohammad Reza Taheri

Mohammad Reza Taheri is a distinguished academic in the Department of Horticultural Sciences and Landscape Architecture at the University of Tehran. With a focus on the application of therapeutic landscape criteria and healing gardens, he has made significant contributions to the design of hospital green spaces. His work emphasizes the importance of nature in health and well-being, particularly in urban environments. As an assistant professor, Taheri’s research and teachings aim to enhance the quality of life through sustainable and therapeutic landscape practices.

Alessio Russo

Alessio Russo is a Senior Lecturer in Landscape Architecture in the School of Architecture and Built Environment at QUT. His research explores the nexus of how urban green infrastructure influences both human health and the delivery of ecosystem services in cities. Prior to joining QUT, Dr Russo held senior academic positions at the University of Gloucestershire, UK (Senior Lecturer and Academic Course Leader), RUDN University in Moscow, Russia (Associate Professor) and FEFU, Vladivostok (Professor and Head of Laboratory of Urban and Landscape Design).

Negin Nasseh

Negin Nasseh is a prominent researcher at Birjand University of Medical Sciences, known for her work in environmental science and engineering. Her research primarily focuses on the synthesis and characterization of novel nanocomposites for photocatalytic degradation of pollutants in wastewater. With a strong publication record, she has contributed to the field with studies on magnetic nanocomposites and their application in removing organic contaminants and antibiotics. Her work is recognized for its impact on cleaner production and environmental pollution control.

Ahmad Taheri

Ahmad Taheri is a master’s student in Landscape Architecture at the Faculty of Agriculture, University of Tabriz. His academic pursuits are centered on sustainable design and the development of green spaces. Taheri’s research interests include urban ecology and the creation of resilient landscapes that support environmental and social health. His dedication to the field is evident through his involvement in various projects that aim to integrate ecological principles into urban planning. As he advances in his studies, Taheri aspires to contribute to the creation of harmonious and sustainable environments that enhance the quality of life for communities.