Growing resilience and promoting health through urban agriculture

During the COVID-19 pandemic, people around the world turned to growing food at home or within their communities. Almost one in five Canadians started food gardening in the past year, with two-thirds of new gardeners influenced by the pandemic. Retailers reported rapid growth in sales and many essential supplies became scarce commodities.

The surge in interest for gardening is part of the larger trend of urban agriculture towards environmental sustainability and social justice, which involves using urban land, labour, and water to produce food. Urban agriculture operations can range from commercial urban farms, to community gardens, backyards, school yards, and rooftop gardens, on any size of urban land, and may also include aquaculture and livestock. Urban farms  are business entities that sell produce regulated through local permits and licenses. Where land access is a challenge, innovations such as skyscraper and rooftop farming (e.g., Singapore) has burgeoned amidst concern about a lack of land and reliance on food imports.     

What are the health benefits of urban agriculture?

Urban agriculture benefits the mental and physical health of residents in numerous ways. Food gardening provides stress relief, connection to nature, social connectivity and recreation in a safe outdoor space, and also produces tangible benefits, thereby enabling people to participate in the food system and connect with how food is produced. As many people faced loss of wages and increasing food prices particularly during the pandemic, urban agriculture supplemented their food supply. In some areas, community gardens were designated as an essential service and urban farms donated food to support local communities. Urban agriculture can provide wildlife habitat to support biodiversity, green infrastructure to aid in stormwater management, and help reduce the urban heat island effect.

Despite the benefits, urban agriculture may present certain environmental health challenges. The following discusses potential public health considerations applicable to urban food gardening.

Is soil in the city safe for urban agriculture?

Healthy soil is characterized by abundant soil nutrients (nitrogen, phosphorus, potassium, and calcium), physical properties like adequate water absorption or drainage, and a neutral pH to allow optimal nutrient absorption. However, even healthy soils can harbour contaminants that may impact human health. Humans can be exposed to contaminants while gardening through dermal exposure, inhalation of dust or volatiles, or ingesting soil particles or contaminated plant tissues. Established gardeners may also have inconsistent knowledge of soil contaminants, and may not be aware of the means to reduce exposure.

There are many potential sources of soil contamination in urban environments. These can include:

• Former industrial or commercial lands, known as brownfields, are sometimes repurposed for urban agriculture, but these soils may contain contaminants from past activities including heavy metals or organic pollutants such as polycyclic aromatic hydrocarbons (PAH’s), polychlorinated biphenyls (PCB’s) used in past industrial processes.

• Deposition of industrial and traffic-related air pollutants, waste disposal, and other human activities can result in a legacy of contamination as well as ongoing sources of pollution. Pollutants such as microplastics that escape waste streams and find their way into soils can be ingested by humans and animals, change soil properties and affect plant function.
• Past residential uses and building construction can also influence pollutant accumulation. Homes built prior to 1990 may contain lead-based paint that can leach into surrounding soils and be taken up into produce.
• Past or present pesticide use or misuse can also threaten food safety, with residues leaching into underlying soils or being retained in crops. Consumers often use excessive concentrations or may apply pesticides during unsuitable weather conditions (e.g., when windy or just before a rainfall), spreading pesticides beyond the intended treatment area.
• The use of treated wood for planters or raised beds may contain creosote that can leach into soils.

Can urban pollution impact produce grown in urban gardens?

Soil contamination is widespread in urban environments, but not all of these pollutants will spread into food. Pollutant uptake depends on soil-related factors, like pH, drainage, and the presence of organic matter, as well as the part of the plant that is eaten. Likewise, certain crops are at greater risk of contamination. Lead accumulates in leafy vegetables and root crops, but less so in fruits of plants (e.g. tomatoes, strawberries). In crops grown near high traffic areas, cadmium was absorbed by tomato leaves. Cadmium deposition in produce can also originate from fertilizer use.

Nevertheless, a recent review of studies of vegetables harvested at urban sites showed low lead uptake, even though lead levels in soils were high. Lead in soil can be diluted by adding compost that nourishes the soil. Other soil remediation methods depend on technologies available.

What are some best practices to identify and address contaminated urban soils?

Although the restoration of brownfield sites is supported by Canada’s Federal Contaminated Sites Action Plan, numerous such sites exist and many have not been remediated to date. Each Canadian province has its own legislation surrounding brownfield site remediation. For example, the Environmental Management Act in British Columbia provides a framework around site identification, assessment, remediation, and liability allocation provisions. Individual or community gardeners concerned with soil contamination can seek municipal guidance and published soil testing guides available in some cities such as Toronto and Vancouver.

Viewing soil as a resource rather than waste can be a point of discussion. Contaminated soil on construction sites is usually designated for a landfill. If the soil is washed and bio-remediated onsite, it may be reused for parkland (vs. transporting in new soil), as was done in the construction of London’s Queen Elizabeth Olympic Park.

Public health can encourage urban gardeners to consider these best practices:

• Visit the site and consult historical land use records. Carry out soil testing if necessary.
• Locate gardens away from heavy traffic, industrial sites, and buildings with lead-based paint.
• Use raised beds containing healthy soil and a non-toxic liner under the bed to prevent leaching of potential contaminants.
• Choose crops that can be picked off a plant, such as corn, tomatoes, and squash.
• Ensure that clean water is available for washing produce on site, and encourage gardens to wash everything before eating.
• Adding clean organic matter (e.g., non-contaminated compost) and maintaining soil pH near neutral can help to reduce the bioavailability of certain metals.
• Educate gardeners about safe pesticide use, proper disposal and advocate for the ban of harmful products.
• Public health can facilitate consumer education on pesticide use, proper disposal and advocate for the ban of harmful products.

Clean soil is one thing – what about clean water?

One of the key factors in preventing soil contamination and growing healthy produce is irrigating with clean water. However, irrigation with potable water can be problematic due to the high demand placed on water systems during the growing season and climate-driven water shortages. Rain barrels are a popular alternative to potable water, but although rainwater is safe for irrigating non-edible gardens, a common concern is whether rainwater stored in rain barrels is safe for food gardening.

Rainwater safety may depend heavily on how rainwater was collected and stored. Rainwater collected from roofs may contain both biological and chemical contaminants. Biological contaminants come from animal and bird feces and include bacteria, cyanobacteria, fungi, and protozoa. Pathogens may flourish in rain barrels if conditions are adequate. For example, rainwater in Ohio contained coliform levels exceeding US standards for secondary recreational water quality, but was acceptable for watering plants. In contrast, rainwater collected around Guelph, Ontario  showed less coliform contamination, especially during colder months, highlighting geographical and seasonal differences that may influence coliform concentration.

Chemical contaminants can be carried by wind or rain and deposited on roofs, or leach directly from roof materials into rain barrels or directly onto soil. These include heavy metals, nutrients, particulates, and pesticides. Roof characteristics including their slope, surface roughness, coating, age, and maintenance history can also affect contaminant levels in the water.

In North America, heavy metals such as zinc and lead are commonly found in rainwater collected from roofs. However, studies of rain barrel contaminants in urban and suburban New Jersey and Philadelphia both found that zinc and lead were well below US federal irrigation standards. Similarly, roof runoff from Puget Sound was free of heavy metals. Cistern-stored rainwater around Guelph, Ontario had low levels of heavy metals. These results indicate that although these contaminants may be present, it is important to consider whether their concentrations are harmful.

Concerns regarding the use of water sources experiencing an algal bloom is fully discussed in our previous NCCEH resource on Irrigating Food Crops with Water Containing Cyanobacteria Blooms.

What are some best practices for using rain barrels?

While not recommended for drinking, rainwater can be used in the production of food crops if certain precautionary measures are followed to reduce the risk of food pathogens and zoonotic vectors. Washington State University Extension offers these tips: 

• Add a “first flush” diverter that directs the initial roof runoff away from the rain barrel.
• Use opaque containers to keep light out and prevent blue-green algae from growing.
• Use a lid or screen to prevent the introduction of mosquitoes, vectors for life-threatening diseases such as West Nile virus.
• Treat rain barrels with bleach regularly to eliminate pathogens or mosquito larvae.
• Set up the irrigation system so that water runs directly on soil and not onto the edible parts of the vegetable.
• Produce collected in the garden should always be washed with clean potable water before consumption.

Summary

Urban agriculture has gained popularity for its potential to increase food security resilience, offer people healthy activity, and provide habitat for a robust ecosystem. Addressing the environmental health risks inherent to urban agriculture will help tip the balance toward these benefits.

Public health professionals can play an important role in urban agriculture through their connections to communities, offer their expertise in health-related issues, and through collaboration with urban planners. Some strategies with which public health can be involved include:

• Educating gardeners on potential soil and water contaminants.
• Assessing environmental health risks (e.g. zoonoses, vermin) in urban animal-keeping policies. e.g. salmonella in backyard chickens
• Advising on potential pest control issues in urban agriculture.
• Educating restaurant, market operators and food banks about the food safety risks of food sourced from urban farms (e.g. salad greens).
• Identifying neighbourhoods lacking fresh food access to establish community garden spaces for local residents.
• Promoting the health benefits of urban agriculture and raise environmental concerns when reviewing land rezoning plans to include urban agriculture.
• Participating in coalitions with community groups, municipal planners, and public health to create regional urban agriculture policy. Examples include allocating public land (e.g. parkland) for urban agriculture, green roof policies for new buildings, integrating community gardens into affordable housing, and establishing urban agriculture as a piece of the larger puzzle of a regional food system and a healthy built environment.

Resources

• Urban Agriculture Garden Guide (City of Vancouver)
• Growing Gardens in Urban Soils (Environmental Protection Agency, US)