5. Food safety considerations for agriculture within urban spaces

At present, over half of the world’s population live in cities, and by 2050, two-thirds of the global population are expected to live in urban areas, with 90 percent of this increase taking place in Asia and Africa (FAO, 2019a). Rapid urbanization and the expansion of cities across the globe (Malakoff et al., 2016) is placing urban food systems in a unique position to help shape the transformation of the overall agrifood systems. While up to 70 percent of all food produced globally is destined for consumption in urban areas (FAO, 2020), urban agriculture is also on the rise in response to growing population in cities. As urban food systems develop, factors like changing demographics, ensuring food security, evolving food preferences, health concerns and climate change will compel greater engagement in issues related to urban agriculture (Knorr, Khoo and Augustin, 2018).

Urban agriculture can be defined as “the growing of plants and raising of animals for food and other uses within and around cities and towns...” (FAO, 2007). Therefore, it encompasses agriculture from both urban and peri-urban contexts. For the purpose of this brief, we focus on agriculture and food production that takes place only within an urban space, i.e. agriculture from an intra-urban perspective.

Urban agriculture or farming can repurpose unused land and space, provide year-round access to fresh food and encourage healthier diets, create employment and livelihood opportunities, and promote affordable food prices (Carbould, 2013; Poulsen et al., 2014). The most important crops of urban farmers tend to be perishable food products and have the locational advantage of being close to the consumers (FAO, 1996). By making it possible to grow food closer to population centres, food miles can be reduced (FAO, 2014; Weber and Matthews, 2008). While the contribution of reduced food miles to the total greenhouse gas (GHG) emissions arising from urban agriculture is still under debate (Weber and Matthews, 2008), the vast majority of GHG emissions are suggested to be mainly attributed to the production and storage phases of food (Mok et al., 2014; Santo, Palmer and Kim, 2016).

Urban farming operations can be of different types and tend to vary by scale. They can be geared towards individual or community consumption, or they can be used for commercial profit (owned by small, medium or large-scale private companies, small-scale family urban farms, community cooperatives, and so on) (Andino, Forero and Quezada, 2021). Urban farms can be found in gardens formed in backyards, rooftops (greenhouses or open-air) and balconies, roadside gardens, community gardens set up in vacant lots and parks, edible walls and indoor farms (Santo, Palmer and Kim, 2016). Open air urban farms can help cool down cities in the summertime, provide valuable habitats for bees and other pollinators, and retain precipitation thereby providing flood-risk mitigation (Dekissa et al., 2021; Rosenzweig et al., 2015; Santo, Palmer and Kim, 2016). Urban agriculture can also include production of non-food plants as well as animal husbandry, beekeeping, aquaculture, and even insect farming for food and feed.

Innovations in indoor farming techniques, where crops can be layered in tiers, are challenging the viewpoint of looking at arable land as one of the metrics for food security (Galeana-Pizaña, Couturier and Monsivais-Huertero, 2018; Park, 2021). Vertical farming and micro-farming (Beyer, 2019), either with soil or soilless using the hydroponic, aeroponic or aquaponic systems, have become popular approaches in mainly indoor forms of urban agriculture.¹⁴,¹⁵,¹⁶,¹⁷ Such farms are pushing the limits of innovation, using technology to digitally monitor and tightly control environments (temperature, light intensity, humidity and nutrient conditions) that allow them to grow food all year-round, while avoiding challenges like erratic weather patterns and pests (Al-Kodmany; 2018; Despommier, 2011). These systems also tend to use less water compared to outdoor farms. For instance, water used in hydroponic farms can be captured and reused rather than being allowed to drain and run-off into the environment. This is especially important in areas where water is already scarce and drought conditions are exacerbated by climate change (Al-Kodmany, 2018).

Urban farms, when designed right, can contribute to improving food security issues in cities (Corbould, 2013). However, there are constraints on the quantity, and depending on the agricultural approach, on the diversity of food that can be grown within urban areas (Clancy, 2016; Costello et al., 2021). A study showed that by dedicating every potentially suitable vacant lot to farming, it would only satisfy the needs of 160 000 people (erstwhile population: 8.1 million) living in New York City, United States of America (Ackerman, Dahlgren and Xu, 2013).

Unlike open farming, some indoor farming setups may need pollination to be carried out manually, which can be labour intensive and costly. In addition, encroachment of expanding cities into the surrounding productive farmlands or areas with wildlife will need to be factored in to weigh the environmental impacts of sustaining urban food production.

A community garden.
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Urban agriculture approaches like vertical farming can be energy-intensive, which not only has environmental ramifications but can also bring economic uncertainties (Love, Uhl and Genello, 2015; The Economist, 2010). Martin and Molin (2019) found that electricity demands, growing medium, transportation and packaging materials all have significant impacts on the environmental sustainability of a vertical hydroponic system. Based on their findings by replacing coir as the growing medium, using paper pots instead of plastic ones, choosing better energy sources such as LED lights powered by solar energy can lead to reductions in the environmental impacts of vertical hydroponic systems. While investing in renewable energy sources would help to lower the carbon footprint of such systems, there may be other trade-offs to consider, for instance, the price of solar energy, and energy backups that may be reliant on fossil fuel, among others. In addition, extreme weather events, exacerbated by climate change, can cause power supply outages, which can be very detrimental to such agricultural systems.

What are the food safety implications to be considered?

As in all food production systems, food safety aspects in an urban food system will need to be considered throughout the farm to fork continuum extending from how the food is produced, stored, packed, sold and consumed. Urban farming is associated with both benefits and challenges when it comes to food safety. Some of these advantages include enhancing traceability, and fewer food miles that can prevent food spoilage and therefore, also reduce food loss (Despommier, 2011). According to published literature, consumers may perceive locally produced food as safer than produce grown elsewhere (Khouryieh et al., 2019).

Indoor urban farms can prevent risks of foodborne illnesses arising from wildlife (deer, birds, feral pigs) having access to produce as it can happen in open fields (Jay-Russell, 2011) and reduce uncertainties of weather, which is becoming more unpredictable due to climate change. A few food safety challenges with urban farming that need to be considered are discussed below.

Concerns arising from soils used in urban agriculture: The location where an urban farm can be set up is a very important food safety consideration as land use in urban areas can leave a legacy of contaminated soil. Therefore, it is important to have knowledge of the history of use of the land where produce will be grown. Multiple contaminants may be found in urban soils at varying levels.

Areas or properties which may have real or perceived contamination issues that pose a serious threat to the safety of food grown there fall under brownfields. These include abandoned gas stations, scrap yards, former factory sites or where older structures have not been demolished correctly, places near dry cleaners, illegal dumping sites, landfills, among others. Sites of former commercial or industrial buildings can be contaminated with asbestos, petroleum products, lead-based paint chips, dust and debris. Older cities tend to have higher levels of heavy metals because of historic use of certain products that contained such chemical hazards. The soil around older homes and under roof drip lines can have higher concentrations of lead from paint and other building materials used on the structures. Air pollution, paint, litter and trash, treatments on wood, coal ash, sewage, and pesticides leave behind various contaminants like heavy metals (such as lead and arsenic), and polycyclic aromatic hydrocarbons (PAHs) as well as antimicrobial resistant microbes that can enter the food chain through urban farming (Defoe et al., 2014; Kaiser et al., 2015; Marquez-Bravo et al., 2016; Nabulo et al., 2012; Säumel et al., 2012; Wortman and Lovell, 2013; Yan et al., 2019; Zhao et al., 2019).

Norton et al. (2013) found that produce grown in open-air farms close to historic mining areas can get contaminated with heavy metals from direct contact with soil. Studies have found that the concentration of heavy metals in urban soils and plants can vary with distance from contamination “hot-spots” such as heavily trafficked roads (Antisari et al., 2015; Werkenthin, Kluge and Wessolek, 2014). While it is difficult to establish a definite quantitative relationship between heavy metal content in soil and in produce, it has been shown that plants can uptake and accumulate heavy metals like lead, cadmium, barium and arsenic from the soil (Augustsson et al., 2015; Izquierdo et al., 2015; McBride et al., 2014). For instance, rice is known to accumulate heavy metals like cadmium and arsenic, both in the plant as well as the grain, which increases risk of exposure to these chemical hazards (Muehe et al., 2019; Suriyagoda, Dittert and Lambers, 2018; Zhao and Wang, 2020). A study by Brown, Chaney and Hettiarachchi (2016) found that lead tends to concentrate mainly in the roots implying that root vegetables like carrots, beets and potatoes may have a higher concentration of lead than produce that is above ground.

Regardless of former use, soils used for urban farming may require testing and if necessary, remediation to lower the concentrations of contaminants to an acceptable level. However, testing for an array of contaminants is not always feasible for urban gardeners. In addition, remediating the soil can be a huge challenge as well. Therefore, some urban farmers tend to remove the old soil, add compost or other regulated soil amendments like biosolids to “dilute” out any heavy metals in the soil, or they sometimes apply phosphate-based fertilizers to reduce bioavailability (Wortman and Lovell, 2013). Sometimes an impermeable barrier is placed on the ground and new soil is added on top. In addition, erecting suitable barriers between urban farms and busy roadways are also advised as a means to keep produce safe from contamination issues.

Other chemical hazards: The warmer microclimates usually found in urban areas (or urban heat island effect) can provide ideal habitats for certain pests (Meineke et al., 2013) prompting growers in open-air urban farms to use higher doses of pesticides to protect their farms. There is currently a lack of studies on the identification and quantification of pesticide residues found in fresh produce grown in urban farms. Overuse of pesticides in the urban environment (from urban farms and general use in residential areas – lawn, turf and home gardens) not only impacts human health through the food chain but also affects the biodiversity in the area and the aquatic ecosystem when the chemicals find their way into the surrounding waterbodies (Meftaul et al., 2020). Many municipalities around the world have regulations to control pesticide applications in urban areas with close proximity to residential locations. Additionally, indiscriminate use of fertilizer or compost application may pollute surface water or storm run-off with excessive quantities of nitrogen and phosphorus, which can potentially exacerbate conditions leading toxic algal blooms in the waterbodies in or near the cities (Wielemaker et al., 2019). However, it must be pointed out that the potential for eutrophication and algal blooms is not unique to urban agriculture. Due to high anthropogenic activity, microplastics can be pervasive in urban environments, soil and atmosphere as well as waterbodies (Evangeliou et al., 2020; Qiu et al., 2020). However, the impact of this pollutant on urban farming and subsequently on human health is still unclear (Fakour et al., 2021; Lim, 2021).

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Certain green leafy vegetables like lettuce can be a source of high levels of dietary inorganic nitrates and can pose possible health risks (EFSA, 2008; FAO and WHO, 2002; Quijano et al., 2017). Application of excessive nitrogen-based fertilizers is one of the major ways nitrates can accumulate in produce (Fewtrell, 2004). However, Jokinen et al. (2022) found that soilless cultivation methods like hydroponic systems have a potential to serve as functional mechanisms to control nitrate content in green leafy vegetables through application of glycinebetaine to the roots of the plants.

Water source: While urban centers are increasing globally, sanitation coverage (collection and treatment) has not kept pace with this growth everywhere (Larsen et al., 2016). Contamination of urban produce (during growth or post-harvest) with pathogenic organisms or chemical hazards from usage of urban wastewater (for irrigation or postharvest cleaning) that is untreated or improperly treated is an important food safety issue (Strawn et al., 2013). A number of foodborne disease outbreaks have been attributed to consumption of fresh produce that was irrigated with wastewater. Apart from foodborne pathogens – different strains of Salmonella, enterohemorrhagic Escherichia coli, Listeria monocytogenes, and viruses such as norovirus – found in wastewater, the issue of antimicrobial resistance can also be exacerbated by the use of wastewater in agriculture (Adegoke et al., 2018; Strawn et al., 2013). Improperly treated wastewater can also be a source of contaminants like pharmaceuticals, as well as act as a reservoir of antimicrobial resistance by being an ideal environment for pathogens to persist. Treatment of wastewater often have limited impact on antimicrobial resistance genes, which do not degrade easily and can be transferred between microbial communities in the environment (horizontal gene transfer) conferring and spreading resistance (Alexander, Hembach and Schwartz, 2020; Mukherjee et al., 2021; Paltiel et al., 2016; Pruden et al., 2006; Zammit et al., 2020).

The quality of water used and its safe reuse in vertical farming systems is a major consideration for ascertaining food safety risks. In an aquaponic system, fish feces can be a potential source of Shiga toxin-producing E. coli. According to Wang, Deering and Kim (2020), Shiga toxin-producing E. coli was found in the water and in the root system of plants, but not detected in the edible portion of the plants. However, if the water tests positive for this microbiological contaminant, it is possible that accidental splashes (during growth or harvest) can lead to contamination of the edible parts of the plant. This is of food safety concern especially if the produce is consumed uncooked. No presence of Listeria spp. or Salmonella spp. were found in the aquaponic or hydroponic systems under study (Wang, Deering and Kim, 2020). In aquaponic systems, sources of microbiological contamination can also be introduced through contaminated fish stocks, through visitors, improper handling measures and through the damaged root systems of plants.

While the safest option in food production is the use of potable or drinking water quality, it is not always a feasible or responsible solution considering increasing water scarcity in many areas. Other types of water can be made fit-for-purpose provided that they do not affect the safety of the final product (FAO and WHO, 2019). Raising awareness among farmers about wastewater use in urban agriculture and the various health risks associated with it will be important for improving food safety in the urban produce food chain (Antwi-Agyei et al., 2015; Ashraf et al., 2013). A publication by FAO lays out some low-cost and low-tech practices that farmers can utilize for wastewater treatment as well as safe irrigation practices that can be adopted to grow food safely (FAO, 2019b).

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Air pollution: Air pollution (ozone, carbon monoxide, sulfur oxides, nitrogen oxides, ammonia, methane, particulate matter, dioxins, heavy metals, polycyclic aromatic hydrocarbons) is increasing in urban areas and can be difficult to control. Urban air quality is affected by a number of anthropogenic factors,¹⁸ such as fossil fuel combustion and greenhouse gas emissions from transportation, agriculture activities, energy supply, industries, among others (Domingo et al., 2021). Climate change also alters the concentration and distribution of air-borne pollutants. While traffic-related air contaminants can disperse quite widely, buildings can act as barriers to focus these hazards in a given area.

Studies have shown that air pollution can reduce the yield and nutritional quality of certain crops grown in urban areas (Agrawal et al., 2003; Thomaier et al., 2014; Wei et al., 2014; Wortman and Lovell, 2013). However, the impact of ambient air quality on open-air urban farming and subsequently on the safety of the food produced is still not fully explored. Certain contaminants like dioxins, heavy metals and polycyclic aromatic hydrocarbons can accumulate in plants and can pose a risk when consumed (Ortolo, 2017). Particulate matters can accumulate on leafy vegetables and act as vectors for other contaminants, such as heavy metals. However, this risk was reduced when plants were thoroughly washed with potable water before consumption (Noh, Thi and Jeong, 2019).

Animal husbandry: Raising animals within urban limits may have food safety implications that are discussed below. However, this activity is more suited for peri-urban areas (Taguchi and Makkar, 2015).

An increasing demand for meat and dairy products, especially in low to middle income countries, combined with lack of sufficient cold-chains can be attributed to the rise of urban livestock farms. Animals such as goats, sheep, cows, pigs, poultry (chickens, ducks) and buffaloes can be found in urban farms across some regions of the world (FAO, 2001). Animal husbandry in cities (on land or offshore) represent an additional source of income through sale of various animal-based food products as well as manure that can be sold for improving urban soil fertility.¹⁹ There are a number of potential hazards to human health that can be associated with urban livestock systems arising from poor hygiene, cramped conditions for keeping animals, flies and parasites that can breed on animal waste, as well as the risk of zoonoses. Backyard poultry can carry foodborne pathogens like Salmonella sp. that can spread to humans if proper hygienic practices are not implemented (News Desk, 2021; Tobin et al., 2015). While most people recover from such illnesses without antibiotics, certain Salmonella strains are increasingly showing resistance to commonly used antibiotics, complicating public health concerns (CDC, 2021; Wang et al., 2019).

Exposure to chemical hazards like dioxins can occur by feeding livestock plant material gathered from the roadside that is heavily trafficked. These chemical hazards tend to accumulate in the fatty tissues of animals and therefore enter the food chain. Inadequate infrastructure in place for animal slaughter, disposal of carcasses, and waste management (removal of manure and urine) can also pose a number of food safety risks to people living in the vicinity as well as to consumers (Alarcon et al., 2017). Proper access to veterinary care and regulations limiting flock or herd numbers in urban spaces are also important considerations.

Vertical fish farming is an emerging approach in aquaculture where fish are reared in a vertical, multi-trophic, mostly closed-loop systems. These structures can be built in urban areas where land is scarce or even offshore (Tatum, 2021). How such systems use and re-use water, treat and dispose effluents from fish and use antimicrobial agents will not only determine the safety of the produced fish but may also influence other public health issues such as potential for eutrophication in nearby waterbodies.

Urban foraging: While this brief focuses on obtaining food by intentionally growing it in an urban space, it will be remiss if gathering or foraging for food in urban areas is not mentioned.²⁰ A clearer understanding of potential safety concerns and nutritional value associated with urban foraging (Stark et al., 2019) is needed as there is a growing recognition that foraged foods are an often-neglected component of urban food systems. More research is needed to determine the extent of exposure to various biological hazards (pathogens and parasites) and chemical contaminants (heavy metals, pesticides and so on) found in urban environments through plants that are collected from both private and public spaces in the urban landscape.

Urban waterways can become polluted from runoff from streets, industrial sites, and gardens. In addition to various waterborne pathogens and parasites, knowledge of which aquatic plants can absorb chemical contaminants like heavy metals from water (Li, Xu and Luan, 2015) is imperative when harvesting them for consumption.

While Gallagher et al. (2020) found lead in urban foraged apples from Boston, United States of America, the level of lead was lower than what the United States Environmental Protection Agency considers safe in a day’s supply of drinking water from the tap. However, systematic evaluation of potential contaminants commonly found in urban landscapes will be needed to adequately address public health concerns.

What is the way forward?

Growing urbanization is driving profound changes in agrifood systems with urban agriculture undergoing rapid development. However, there is inadequate research on potential human health risks arising from consuming food specifically produced within urban spaces. Improved availability of fit-for-purpose land/space and water, access to markets, greater capital and operating funds, opportunities for technical training to improve the knowledge base of urban producers and their agricultural skills, and development of appropriate regulatory frameworks and strategies are some of the areas that also determine the success of urban agriculture. Greater attention also needs to be paid to infrastructure for hygienic intra- and interurban processing, storage and transportation, as well as integration of urban food production into urban planning to ensure land allotments are at a safe distance away from main roads and other contamination sources to facilitate safe food production in urban areas.

Further development of urban agriculture entails increasing access to land which may propel efforts to remediate and rejuvenate brownfields. However, turning brownfield sites into areas that are safe and suitable for food production is often not straightforward and requires greater engagement with municipal authorities and landowners, regular monitoring for contaminants in these spaces, and knowledge dissemination among the public (Miner and Raftery, 2012).

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The concept of creating a “circular city” is gaining attention whereby various organic disposals from different processes are repurposed as resources to promote agricultural productivity in urban areas (Ellen MacArthur Foundation, 2019; Skar et al., 2020). However, care must be taken to ensure that the inputs into such a closed-loop bioeconomy are safe to use and that sources of contamination are not introduced as this can facilitate concentration of contaminants if adequate monitoring and treatment procedures are not put in place.

Advancements in digital technologies may allow urban farmers to “farm from afar” whereby multiple urban farms can be accessed remotely, for instance to tweak conditions – soil pH value, nutrient level, light intensity, among others – as needed or even to sound alarms if manual interventions are required. Digital innovations may also facilitate periodic testing for foodborne pathogens at various points in vertical farms as well as enhance traceability mechanisms to enable identification and removal of contaminated produce before it becomes a public health issue.

In order for cities to foster inclusive, nutritious, safe and sustainable urban food systems and to effectively address challenges, good governance (mechanisms, capacities, policies, financial support) specific to urban food systems will be needed. This is a transdisciplinary area that needs multisectoral engagement from local governments, civil society, the private sector as well as municipal, provincial and national governments (Knorr, Khoo and Augustin, 2018; Ramaswami et al., 2016; Tefft et al., 2020). However, lack of suitable regulatory frameworks to govern urban agriculture has been identified as a barrier for market expansion in different studies (FAO, 2012; Sarker, Bornman and Marinova, 2019). Regulation of urban agriculture would require considerable resources, and currently many LMICs lack the infrastructure and institutional framework to monitor it (Merino et al., 2021)