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Review

Urban Gardening—How Safe Is It?

by
Katalin Hubai
,
Nora Kováts
* and
Bettina Eck-Varanka
Centre for Natural Sciences, University of Pannonia, Egyetem str. 10, 8200 Veszprém, Hungary
*
Author to whom correspondence should be addressed.
Urban Sci. 2024, 8(3), 91; https://doi.org/10.3390/urbansci8030091
Submission received: 27 May 2024 / Revised: 15 July 2024 / Accepted: 17 July 2024 / Published: 19 July 2024
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Abstract

:
Urban gardening has become more and more popular in recent years, as people might prefer to grow their own vegetables from controlled sources. In addition, community building also plays a key role. However, air pollution in settlements is a serious hazard affecting the quality of home-grown vegetables. During the vegetation period, traffic is the main factor generating atmospheric particulate matter. These particles will, in turn, bind to potentially toxic compounds, of which heavy metals and polycyclic aromatic hydrocarbons (PAHs) are the most widely studied and discussed. In addition to their potential toxicity, both groups contain carcinogenic species. Heavy metals, as well as PAHs, are capable of bioaccumulation, depending on the element or compound’s characteristics and the vegetable species. Some leafy vegetables can accumulate these toxic materials in significant quantities. As dietary uptake is considered the major exposure route of both heavy metals and PAHs, the consumption of impacted vegetables might even pose human health risks. This recent review summarises available data reported on heavy metal and PAH accumulation in urban environments, specified by vegetable species. Also, the assessment of possible human impact is given.

1. Introduction

Urban gardening has become more and more popular in recent years. The purpose of gardening includes, among others, general relaxation and physical activity, as well as connection with friends or the whole community [1], enhancing social cohesion [2]. The positive effects on mental wellbeing become more obvious in late adulthood [3]. Also, such positive effects were proven to be even more characteristic during the COVID-19 pandemic [4].
Several studies have assessed the potential positive influence of gardening on physical health. Veldheer et al. [5], for example, reported better cardiovascular health status among older adults. Urban garden spaces also increase diversity, in both aesthetic and ecological terms [6]. During urban gardening, new locations can be exploited for cultivation, thereby increasing food supply sources [7]. Under pandemic conditions, the following new benefit emerged: food security, that is, to obtain access to fresh food sources [8].
In addition to the reportedly positive influence on the mental and physical health of urban gardeners, people naturally wish to eat what they grow. In general, community gardening is supposed to increase fruit and vegetable intake [9]. Lewis et al. conducted a study on defining the targeted outputs of urban gardening and specified ’safer food’ and ’self-produced food’ as important aspects. In the interviews, several gardeners stated the importance of knowing ‘what went into the vegetables’, implying that they were not treated with synthetic chemicals [10]. A Croatian study conducted in Zagreb emphasised the benefits of home-grown produce, including lower prices (in comparison to shops or markets) and better flavour [11]. Food security maintenance can also be important to address climate change adaptation [12]. Non-food-oriented motifs also include a physical connection with vegetables, as a French study conducted in Montpellier determined [13]. As part of urban agriculture, rooftop gardens have gained special attention, as rooftops are typically unused surfaces but can be turned into effective agricultural spaces [14].
On the other hand, the consumption of home-grown vegetables in urban environments might pose some risk of contamination, which cannot be neglected [15]. It has been shown that dietary uptake is the major exposure route of heavy metals [16] and polycyclic aromatic hydrocarbons (PAHs) [17]. These two groups are commonly associated with air pollution. This review addresses the problem of air quality and the potential risk of accumulation in commonly cultivated vegetables or fruits in urban gardens.

2. Urban Air Quality

Urban air quality has become a major concern in the whole world, both in developed and developing countries. Air pollution is generally defined as a situation in which ‘substances in the atmosphere exceed a certain concentration’ [18]. Emissions are basically influenced by the drastic growth in urban populations, as according to United Nations statistics, over half of the world’s population lives in urban areas (United Nations 2018). This tendency will most probably continue in the future.
Atmospheric particulate matter (PM) is a significant component of air pollution and is considered a major environmental risk [19]. In Europe, the threshold levels for 10 μm-diameter (PM10) and 2.5 μm-diameter (PM2.5) particles have been set (European Parliament C. Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe; 2008). PM2.5 concentrations are considered the best proxy for assessing the level and risk of air pollution [20] and are suggested as objective indicators [21].
The increasing volume of motorised traffic is an important contributor to air pollution problems in cities. Approximately 50% of the particulate matter in urban air is generated by urban traffic [22]. Two main sources apply, the road transport of goods and the private purchase of family cars.
In addition to traffic-related sources, biomass burning is also an important source of PM. Biomass is generally used for heating during the cold season; naturally, it will not have a potential effect on air quality in the vegetation period. However, it has been reported that burning agricultural residues on the outskirts of cities can have a serious influence on urban air quality [23].
Particles generally bind to potentially toxic compounds and elements, of which PAHs and heavy metals have been the most widely studied [24]. The smaller the worse; fine particles with diameters of 2.5 μm and below (PM2.5 and smaller) can bind to relatively more of these contaminants due to the greater relative surface area [25].
PAHs are unsaturated hydrocarbons containing at least two infused aromatic rings. They are ubiquitous environmental pollutants that are recognised as persistent organic pollutants (POPs) by the Stockholm Convention (Stockholm Convention 2011). In urban areas, the main anthropogenic sources include winter heating (coal and biomass burning) and traffic-related emissions [26], especially emissions from diesel-powered vehicles. During the vegetation period, traffic-related sources provide the main input. In Europe, diesel-powered vehicles still pose an important emission factor, despite more and more widely utilised technological improvements.
Sixteen priority PAHs are listed by the US Environmental Protection Agency (EPA) (EPA-PAHs) as demonstrating the highest environmental risk [27]. Of these, the group of so-called Car-PAHs was defined [28]. These PAHs are listed in Table 1.
In addition to well-known toxic properties, several PAHs show carcinogenic potential. The International Agency for Research on Cancer (IARC) reviewed experimental data for 60 individual PAHs and identified the following three groups: carcinogenic to humans (Group 1), probably carcinogenic to humans (Group 2A) and possibly carcinogenic to humans (group 2B) [29]. Benzo[a]pyrene is classified as ‘carcinogenic to humans’ (Group 1). In addition to individual PAHs, diesel engine exhaust was classified as carcinogenic to humans by the International Agency for Research on Cancer [30]. In 2016, the IARC even classified particulate matter (PM) and outdoor air pollution as carcinogenic to humans (Group 1) [31].
Traffic will have different contamination sources, in addition to tailpipe emissions. While exhaust emissions keep diminishing, the so-called non-exhaust emissions will outdo them, at least in the OECD countries [32]. These include road dust resuspension, tyre wear, road abrasion and brake wear. In contrast to emission-related contaminants, these factors are the major sources of heavy metals such as Cr, Pb, Zn and Cu, all of which have well-known toxic potentials [33]. Földi et al. experimentally proved that vehicular traffic is the major source of the potentially toxic antimony (Sb) in urban soils, as this metal is an important component of brake linings, substituting asbestos, as its use had been banned in the European Union [34].
In general, atmospheric deposition is considered a major source of heavy metals in agricultural systems [35]. Regarding atmospheric fallout, the heavy metals that are most often addressed are cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) [36]. Cd and Pb are toxic, and the excessive dietary intake of vegetables contaminated with these elements might cause human health risks [37]. Zn and Cu are essential elements, but excessive concentrations in crops might result in toxicity to humans [38,39].
Pb was previously associated with the usage of leaded fuels (gasoline with PbO4 addition). As this kind of fuel was phased out in most developed countries, the main sources of Pb nowadays are tyre wear and lubricating oils [40]. As road dust may be washed out by surface runoff, heavy metal contamination, including Pb, can be transported to nearby (urban) soils [41].
Clarke et al. measured the concentrations and bioavailability of Pb, As and Cd in soil from twelve community gardens in Los Angeles County (California, USA). The concentrations of all three elements were increased with the proximity of roads, suggesting vehicular traffic as the main source [42].
Harada et al. measured heavy metal concentrations in the soil of a rooftop vegetable farm (Brooklyn Grange, USA). The study intended to assess the atmospheric deposition of heavy metals such as Pb, Mn, As, Ba, Cd, Cu and Zn but concluded that the concentrations of these elements in the soil never exceeded established guidelines [43]. On the contrary, an Australian programme called VegaSafe was launched in 2013 and was intended to provide free chemical analysis for soils in vegetables of urban gardens. The results revealed that approximately 35% of homes provided soil samples in which lead concentrations exceeded the Australian residential guideline (300 mg/kg Pb) [44].
However, in addition to atmospheric deposition, urban soils may receive heavy metal input from many other sources, such as industrial contamination e.g., [45] or waste disposal [46]. The possibility of natural (geogenic) origin has also been proven [47]. Regardless of the contamination source, Schram-Bijkerk et al. suggest carrying out an analytical evaluation of the soil, and in case of the presence of toxic pollutants, the cultivation of leafy vegetables should be avoided, according to the authors [48].

3. Uptake of Atmospheric Pollutants

The uptake of PAHs and heavy metals by plants can occur directly from the polluted air or indirectly from the soil, which receives atmospheric fallout. However, the exposure pathways of PAHs and heavy metals differ to some extent. Figure 1 summarises the uptake processes.
Considering the uptake of PAHs, low-molecular weight species typically occur in gaseous form, while higher-molecular weight PAHs (compounds with more than four aromatic fused rings) are bound to fine particles [49,50]. Plants are exposed to both phases and can uptake PAHs of different molecular weights, although the efficiency and pattern of accumulated PAHs differ depending on the taxon in question. The mechanism may also differ; gaseous PAHs can enter directly through the stomata; they can also diffuse through the wax and cuticular lamellae into the interior parts of the leaves [51]. The uptake from the gas phase is considered a more important exposure pathway than the particle deposition onto plant surfaces [52]. Quéguiner et al. conducted a simulation study for the uptake mechanism of atmospheric benzo(a)pyrene by leafy vegetables and concluded that accumulation occurs as a result of direct atmospheric deposition, excluding soil-to-plant transfer [53].
Particles and, naturally, particle-bound contaminants can be deposited both on plant surfaces and onto the soil. PAHs first desorb from particles that are deposited on the surfaces of the leaves [54] and then are transported to the cuticular wax, representing the primary pathway of leaf accumulation [55].
In addition to the direct deposition of particles on soil or plant surfaces, wet deposition is also an important pathway [56]. Wet deposition implies that both gas and particle phase PAHs can be washed out by rain or snow. This process also provides an important pathway for PAH removal from the atmosphere. Its magnitude, however, will depend on the precipitation amount, which might show significant spatial inequalities and seasonal patterns [57]. It is important to note that this process has been proven efficient for all particle-associated PAHs, including higher-molecular weight (HMW) compounds [58].
The foliar uptake of heavy metals can occur through stomata, cuticular cracks, lenticels, ectodesmata and aqueous pores [59]. According to transmission electron microscopy observations of Li et al., penetration through the leaf cuticle proved to be the primary pathway [60]. Figure 2 illustrates the relative size of PM2.5 to the stomata of Lycopersicum esculentum (tomato) leaves. Gao et al. conducted a field experiment to reveal the mechanisms via PM2.5-bound Pb accumulation in cabbage leaves. Scanning electron microscopy revealed that the stomatal apertures were wide and long enough to allow PM2.5 particles to enter the leaves, carrying contaminating lead [61]. The foliar uptake of atmospheric Cd and Pb was demonstrated by Gajbhiye et al. [62].
The potential to uptake and accumulate particle-bound toxic compounds is directly influenced by leaf morphology e.g., [63,64], as morphological traits may determine the interaction between particulate matter and plant leaf surfaces. Important traits are the shape and size of the leaf; plants with high surface-to-volume ratios accumulate more organic air pollutants than species with compact leaves. Leaves that have larger surface areas will have better accumulation potential than those with smaller areas [65]. Palmately lobed leaves are also more efficient in capturing particles, as they probably create more turbulence in the boundary air layer with their complex shape [66].
Uptake and accumulation are also determined by epicuticular wax content. On one hand, the glossy and smooth surface of the leaves can prevent the retention of particulate matter [67]. On the other hand, the wax layer might effectively store toxic compounds. Margenat et al. found that relatively high lipid content assists accumulation. PAHs are lipophilic organic pollutants that are expected to accumulate in leaves that contain more extractable lipids, such as epicuticular wax + tissue lipids [68]. Sæbo et al. reported that the total particulate matter accumulation was increased in parallel with the increase in the quantity of leaf wax, independently from the size of the PM [69].
Trichomes might also play a significant role in regulating the uptake of PM-bound contaminants. Gao et al. studied the atmospheric Pb uptake of Chinese cabbage (Brassica rapa ssp. pekinensis) and reported that PM2.5-bound Pb, in addition to passing through open stomata, entered the cellular space of trichomes and accumulated in the basal compartment [70].
It is generally assumed that leafy vegetables will absorb more potentially toxic contaminants [71]. Leafy vegetables, such as different cabbages, lettuces, spinach, parsley, etc., make up a significant share of varieties cultivated in urban gardens, depending on the geographic region, climate and season [72]. Lettuce and kale are especially recommended in urban rooftop gardens, as they can be maintained in relatively shallow soil [14]. Comparing different vegetable species, lettuce and cabbages are perhaps at the highest risk. Lettuce has a high foliar surface and thin cuticula, which favour atmospheric accumulation [73,74]. According to Leitão et al., lettuce and cabbage are the most popular vegetables cultivated in urban gardens in Lisbon (Portugal), and they are very tolerant to contamination but show no visible signs of damage, in spite of relatively high amounts of accumulated toxic compounds [75]. Lettuce and kale have also been proposed for biomonitoring purposes due to their heavy metal uptake capacities [75,76]. It should also be noted that the careful selection of potentially less sensitive varieties will reduce the risk of accumulation [77].
Other leafy vegetables and culinary herbs, such as Pimpinella anisum, Spinacia oleracea, Amaranthus viridis, Coriandrum sativum and Trigonella foenum graecum, were found to contain heavy metals in an Indian study [78]. Augustsson et al. also detected high accumulation in the case of parsley [79].
It should be noted, however, that other vegetables can also be impacted via atmospheric pathways. Engel-Di Mauro, for example, detected considerable atmospheric uptake and accumulation in tomato fruit samples collected in urban vegetable gardens in Rome (Italy) [80].
The deposition of particles on the soil surface will transfer potentially toxic compounds, which will be available via the soil–root pathway [81]. Jia et al. demonstrated that soil absorption contributed to nearly 10% of total PAH uptake in tested vegetables [82]. PAHs can be transported via the plant vascular system or by diffusion through the cells [83]. This transport mechanism, however, is more efficient for low-molecular weight PAHs [84].
Considering the importance of the soil–root pathway in the accumulation of heavy metals in the above-ground parts of edible plants, some authors assume that it is the major pathway [85]. Kabata-Pendias summarises that plants readily take up metals dissolved in the soil solution regardless of the form (ionic, chelated or complex forms) in which they occur [86]. Zhou et al. experimentally analysed the bioavailability of deposited Cu and Pb, reporting that approximately 61% of the Cu and 76% of the Pb depositions can be potentially mobilised and taken up by plants [87]. Heavy metals can accumulate in significant concentrations in root vegetables such as chard, radish, carrot, potato or even taro (reviewed by Bidar et al. [85]). Heavy metals, however, have also been shown to be translocated from the roots to the edible parts of leafy vegetables such as lettuce or cabbage [88].

4. Bioaccumulation of PAHs

Actual data on PAH accumulation in different vegetables fall into a relatively wide range. Considering lettuce, Gelman, for example, reports that practically no accumulation was detected in rooftop gardens in Helsinki [89].
Amato-Lourenco et al. experimentally exposed two leafy vegetables (Brassica oleracea var. acephala (collard greens) and Tetragonia tetragonioides (New Zealand spinach)) in nine urban community gardens of Sao Paulo (Brazil). The exposure was set as 45 days, which normally corresponds with the average lifecycle of both species. The study revealed that PAH accumulation was negligible, except in one case; the DBahA concentration was as high as 7.4 μg kg−1 in spinach leaves in a garden that was located < 15 m from a very heavily trafficked street [90]. Tusher et al. compared concentrations of PAHs in red amaranth (Amaranthus cruentus) and spinach (Spinacia oleracea) grown on the rooftops of the urban and peri-urban areas of Tangail, Bangladesh. The only PAH detected in the samples was two-ring naphthalene, with average concentrations of 1.24 mg/kg in red amaranth and 2.25 mg/kg in spinach [91].
Kováts et al. conducted a pot study to assess PAH accumulation in lettuce. Pots were placed in gardens of villages of different sizes, including a medium-sized settlement affected by heavy traffic. The concentration of total PAHs was in the range of 9.1–186 μg kg−1 in the samples after 2 months of exposure. The study included one sampling spot relatively close to a local railway. This spot produced a sample with a rather high percentage of five-ring PAHs [92]. Five-ring PAHs include benzo(k)fluoranthene, benzo(b)fluoranthene and benzo(a)pyrene, which are associated with vehicular emissions [93]. This sample also contained the six-ring benzo[g,h,i]perylene, which is considered a marker of gasoline exhaust emissions [49]. The study also assessed the magnitude of PAH bulk atmospheric deposition by measuring PAH concentrations in the upper 5-cm layer of the pots. The total PAHs were in the range of 31.6 and 595 μg kg−1, indicating that atmospheric deposition can provide a non-negligible contribution to soil PAH content.

5. Bioaccumulation of Heavy Metals

Similarly to PAH uptake, the accumulation of heavy metals is supposed to depend on the crop species, the part of the vegetable and the variety. Table 2. summarises studies dealing with the actual accumulation of selected heavy metals.
Several studies discuss that in urban environments, Pb presents the highest risk. Samsoe-Petersen et al. reported that in Copenhagen, the lead content of vegetables grown in highly Pb-contaminated urban soils was above acceptable levels, while other metals did not pose such a risk [94]. Another study conducted in Copenhagen approximately one decade later found no evidence of any risk of consumption of urban vegetables, even when they were grown in contaminated soils. The main drawback of such studies is that they do not distinguish between pollution sources; therefore, it is practically impossible to find out if the soil pollution was caused by atmospheric deposition, either directly or indirectly [95].
The number of studies directly addressing heavy metal uptake, either from the soil or directly from the atmosphere as a result of atmospheric exposure, is much more limited, although it is widely supposed that the main source is road traffic [96]. Säumel et al. measured heavy metal concentrations in the inner city gardens of Berlin (Germany) to determine the potential effects of traffic-related exposures. The main finding was that the trace metal contents of inner city samples exceeded the values measured in samples from the supermarket, especially in the case of Cr, Cd, Zn, Pb and Cu. The study also revealed that heavy metal content depended on traffic intensity, such as the proximity of roads or the number of cars passing. The negative influence could be reduced by the presence of barriers between roads and gardens (such as buildings) [97]. Similarly, the effect of road traffic was demonstrated in the study by Antisari et al. The heavy metal contents of different vegetables were measured, comparing rural and urban samples. The 3-year study revealed that crops grown in the proximity of roads accumulated significant amounts of heavy metals, reaching as much as 210 mg/kg dw for basil. The following heavy metals were most often detected in the samples: As, Ba, Cu, Pb, Sb, Sn, V and Zn [98].
Amato-Lourenco et al. assessed the accumulation of atmospheric heavy metals in a pot study conducted in community gardens in Sao Paulo, Brazil, with special regard to elements associated with traffic, such as Al, Cr, Mn, Fe, Ni, Cu, Zn, As, Rb, Cd, Ba and Pb. The study was designed to assess temporal and spatial patterns, comparing the accumulation capacity of Brassica oleracea var. acephala (collard greens) and Spinacia oleracea (spinach). After a longer exposure time, such as 60 days, significant accumulation was experienced; the concentrations of Pb and Cd in both vegetables exceeded the limit values for consumption. An analysis of the spatial pattern revealed the link between traffic intensity and accumulated concentrations [99].
Experimental pot studies have also demonstrated that crops are able to accumulate Pb from the atmosphere [100].
Pandey et al. assessed the accumulation of heavy metals as a result of atmospheric deposition in different zones of Varanasi (India). In atmospheric particles, the highest concentrations were measured for Pb, followed by Cu, Ni, Cr and Cd. Considerable accumulation was found in spinach leaves, reaching as much as 18.21 µg/g for Pb, 21.91 µg/g for Cu, 11.53 µg/g for Cr and 4.91 µg/g for Cd [101]. A similar study also conducted in Varanasi compared the accumulation capacity of different vegetables, such as Abelmoschus esculentus, Beta vulgaris and Brassica oleracea. The heavy metal accumulation patterns differed; Zn and Cu were accumulated in the highest concentrations in Brassica oleracea, Cd was accumulated in the highest concentration in A. esculentus and B. oleracea, and Pb was accumulated in the highest concentration in Beta vulgaris. The results also revealed that Cu and Cd posed human health risks via the consumption of all three vegetables tested, while Pb was accumulated exceeding the safe limit only in B. oleracea [102].
Rodríguez-Rodríguez et al. exposed strawberry plants to synthetic airborne particles in a closed chamber experiment to assess heavy metal accumulation. The results revealed that strawberry fruits accumulated potentially toxic elements such as Cd, Ni and Cr in the interior part, most probably via uptake by their stomata or cuticles. The authors also stressed that accumulation studies concerning strawberries can be important due to the popularity of this crop in urban gardens [103].
The accumulation of atmospherically deposited cadmium (Cd) was assessed in the study by Ouyang et al. using water spinach (Ipomoea aquatica Forsk) and pak choi (Brassica chinensis L.) leaves. Cd accumulation was demonstrated in the cell walls, also highlighting the potential health risks of consuming Cd-containing vegetables [104].
Table 2. Studies reporting heavy metal accumulation in urban gardens.
Table 2. Studies reporting heavy metal accumulation in urban gardens.
Heavy MetalReference
Cr, Cu, Mn, Ni, Pb, ZnGupta et al., 2013 [78]
As, Ba, Cd, Co, Cr, Cu, Ni, Pb, Sb, Si, Ti, ZnAugustsson et al., 2023 [79]
As, PbEngel-Di Mauro, 2018 [80]
As, Cd, Cr, Cu, Ni, Pb, ZnSamsoe-Petersen et al., 2000 [94]
As, Cd, Cr, Cu, Ni, Pb, ZnWarming et al., 2015 [95]
Cd, Cr, Cu, Ni, Pb, ZnSäumel et al., 2012 [97]
As, Ba, Cd, Co, Cr, Cu, Ni, Pb, Sb, Sn, V, ZnAntisari et al., 2015 [98]
Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Rb, ZnAmato-Lourenco et al., 2016 [99]
Cd, Cr, Cu Ni, Pb Pandey et al., 2012 [101]
Cd, Cu, Zn, PbSharma et al., 2008 [102]
As, Cd, Cr, Ni, PbEsther Pérez-Figueroa et al., 2023 [105]
As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, ZnRossini-Oliva and López-Núñez, 2021 [106]
Co, Cr, Cu, Mn, Ni, Pb, ZnIzquierdo et al., 2015 [107]
Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, P, Pb, Rb, Se, V, ZnIzquierdo-Díaz et al., 2023 [108]
Cd, Pb, ZnZiss et al., 2021 [109]
Ba, Cd, Co, Cr, Cs, Cu, Fe, Mn, Ni, Pb, Rb, Zn Sussa et al., 2022 [110]
PbSung and Park, 2018 [111]

6. Human Health Risk of Consuming Contaminated Vegetables

Crops grown in urban gardens can accumulate atmospheric heavy metals or PAHs; the question is whether these accumulated quantities pose actual harm to consumers. Buscaroli et al. suppose that the risks can be underestimated [112]. Unfortunately, most studies calculate the overall risk for vegetables grown in urban gardens and do not differentiate between uptake from contaminated soil or from the atmosphere [105,106]. Ziss et al., for example, conducted a citizen science project in Vienna (Austria) to evaluate the possible accumulation of Pb, Cd and Zn in radishes and spinach. The results revealed that some spinach samples contained Pb in concentrations above safe limits; however, the source of excess Pb could have been soil contamination [109]. Data addressing the problem of atmospheric uptake that can result in bioaccumulation actually damaging the health of consumers are rare.
The available studies, however, generally suggest negligible risk. The human health risk of consuming vegetables produced in urban community gardens in Madrid (Spain) was calculated based on the bioaccessibility of the accumulated metals [107]. The study reported that such a risk fell into the acceptable category. Izquierdo-Díaz et al. conducted a pot study in Copenhagen (Denmark) to assess atmospheric heavy metal uptake in lettuce. The study concluded that vegetables grown in a clean substrate can be considered safe for consumption, as accumulated concentrations were within limit values [108]. A similar conclusion was drawn by Sussa et al. when the concentrations of non-essential elements such as Cd, Cu, Ni, Cr, Co and Pb were measured in lettuce cultivated on a rooftop urban garden in the metropolitan region of Sao Paulo [110]. It should be noted, however, that in exceptional cases, such as urban gardens situated near industrial areas, accumulation can exceed safe limits. Sung and Park reported that industrial fallout in the city of Daejeon (Korea) resulted in Pb contamination above the Korean standard for maximum Pb content (300 μg/kg in leafy vegetables) in 25% of the samples taken. The study also stressed that local gardeners still considered their product ‘safe’ [111].

7. Concluding Remarks

Due to numerous reasons, more and more people find satisfaction in urban gardening. Taking into consideration that over half of the world’s population lives in urban areas, the share of home-grown vegetables in the human diet seems to increase, as well. While the positive effects cannot be questioned, crops grown in urban gardens can be exposed to a wide range of atmospheric pollutants directly from the air or indirectly via the air-soil–root pathway. Atmospheric contaminants, such as polycyclic aromatic hydrocarbons and heavy metals, show a well-known tendency to accumulate in crops and are potentially transferred to humans consuming such vegetables. While some studies report that accumulation in agricultural crops grown in contaminated environments can actually pose human health risks, the number of papers reporting on vegetables cultivated specifically in urban gardens is very limited. These papers generally suggest that these vegetables can be quite safely used, or at least that the concentration of accumulated contaminants is normally within the limit values.

Author Contributions

Conceptualization, K.H., N.K. and B.E.-V.; methodology, K.H., N.K. and B.E.-V.; resources, N.K.; writing—original draft preparation, N.K.; writing—review and editing, N.K.; visualization, K.H.; supervision, K.H.; project administration, K.H.; funding acquisition, N.K. and B.E.-V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the NKFIH-872 project ‘Establishment of a National Multidisciplinary Laboratory for Climate Change’.

Data Availability Statement

All data analysed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

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Urbansci 08 00091 g001
Figure 1. Uptake mechanisms of atmospheric PAHs and heavy metals. LMW: low molecular weight, HMW: higher-molecular weight PAHs.
Figure 1. Uptake mechanisms of atmospheric PAHs and heavy metals. LMW: low molecular weight, HMW: higher-molecular weight PAHs.
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Figure 2. PM2.5 particles around stomata of Lycopersicum esculentum (tomato) leaves. Photo was taken by Thermo Scientific™ Talos™ F200X scanning transmission electron microscope (by courtesy of Ms Zsófia Békéssy).
Figure 2. PM2.5 particles around stomata of Lycopersicum esculentum (tomato) leaves. Photo was taken by Thermo Scientific™ Talos™ F200X scanning transmission electron microscope (by courtesy of Ms Zsófia Békéssy).
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Table 1. List of EPA-PAHs that are Car-PAHs. Car-PAHs are given in italics.
Table 1. List of EPA-PAHs that are Car-PAHs. Car-PAHs are given in italics.
PAH
Naphthalene
Acenaphthylene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Benzo(a)anthracene
Chrysene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(e)pyrene
Benzo(a)pyrene
Indeno1,2,3CD-Pyrene
Dibenzo[a,h]anthracene
Benzo(g,h,i)perylene
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Hubai, K.; Kováts, N.; Eck-Varanka, B. Urban Gardening—How Safe Is It? Urban Sci. 2024, 8, 91. https://doi.org/10.3390/urbansci8030091

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Hubai K, Kováts N, Eck-Varanka B. Urban Gardening—How Safe Is It? Urban Science. 2024; 8(3):91. https://doi.org/10.3390/urbansci8030091

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Hubai, Katalin, Nora Kováts, and Bettina Eck-Varanka. 2024. "Urban Gardening—How Safe Is It?" Urban Science 8, no. 3: 91. https://doi.org/10.3390/urbansci8030091

APA Style

Hubai, K., Kováts, N., & Eck-Varanka, B. (2024). Urban Gardening—How Safe Is It? Urban Science, 8(3), 91. https://doi.org/10.3390/urbansci8030091

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