Avoiding Contamination in Urban Environments
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One of the joys of living in a city, for me, is spotting and observing wildlife that calls this environment home. This includes animals like squirrels, hawks, wild turkeys, deer, and foxes, but also plants, insects, fungi— any living thing that dwells here of its own accord. Even things planted by humans— such as trees in a park, veggies in a garden, or wildflowers in a restoration site— bring me feelings of happiness.
This, of course, extends to foraging as well. I love finding plants toughing it out in sidewalk cracks or vacant lots, little bits of wilderness in a human-dominated landscape. However, just because I enjoy seeing them doesn’t necessarily mean that I harvest and eat them! Some plants are better left alone, as the soil they grow in may be contaminated, posing a health threat to anyone who would consume them.
Thankfully, there are a few guidelines to avoiding contamination and enjoying the harvest of an urban environment!
Know Your Soil
One way to avoid contamination is to know the location of toxic sites exist near you. You can find these through your local pollution control agency; the Minnesota Pollution Control Agency has a public database of toxic sites in the state, and a list of Superfund sites. (Superfund sites are major hazardous waste sites that receive federal money from the EPA to clean up the contaminants.)
Another way to avoid contamination is to learn to recognize general areas that are at risk. These include, but are not limited to:
Areas along sidewalks (from dog poo)
Tracts along old houses (from lead paint)
Roadsides (from car exhaust)
Railroad tracks (from herbicides sprayed to clear tracks as well as hazardous items carried by train cars)
Golf courses (from chemical pesticides)
Industrial sites (from hazardous waste, leaks, or burning)
How much space to leave between your foraging activity and the risky site is a matter of debate. I have read anywhere between 10 feet and 100 feet. Unfortunately, these recommendations don’t seem to be based on any scientific data. This is probably at least partially due to the lack of research into human health effects of foraging. However, we can look to agricultural studies for guidance. According to a study regarding the safety of urban-grown vegetables in Berlin, “depending on local conditions such as slope, wind and building structures, the influence of roads as pollution sources extends from a few metres to kilometres.” With such a wide safety range, what is the urban forager to do?
First of all, don’t panic. Heavy metals and other contaminants are found in urban areas as well as rural areas, and are present in food grown in cities, countrysides, and food bought at the supermarket. The key is not to exceed certain levels. Of course, without access to a laboratory, we can’t know exactly what those levels are. But we can take educated guesses.
If you have your own yard or garden, you probably have a pretty good idea of how healthy the soil is. You will know whether you not you spray toxic chemicals, if you have a pet who poos in the yard, and hopefully you’ll know if your house contains lead paint and have some history of the lot it’s built on. If you are worried about soil contamination, you can have your soil tested by a laboratory. A list of places that test soil in the Twin Cities Metro Area can be found here. Basic tests for lead and nutrient levels can be done at the University of Minnesota Soil Testing Laboratory. If you have a space in a community garden, the soil there will have been tested and found safe. (This is the policy in Minneapolis, anyhow.)
For those of us who don’t have our own yard or who would like to extend our foraging range beyond it, we can follow these guidelines:
The farther you forage from a hazardous site, the better;
Barriers such as buildings, walls, and hedges reduce the overall contamination level;
Lower traffic densities are generally safer than higher ones.
Certain Parts are Safer
How are contaminants ingested? It is typically not by eating plant parts that have absorbed the contaminant; rather, it is by eating contaminated soil or dust that has blown onto the plant. Therefore, washing your harvest before you eat it goes a long way towards minimizing risk.
However, in some instances it is important to consider what part of the plant you are eating— root, leaf, steam, fruit, nut, etc. The aforementioned study about the safety of urban-grown vegetables in Berlin concluded that “a number of crop samples from inner city sites had trace metal contents many times higher than the samples from the supermarket…. With the exception of cadmium, the elevated trace metal concentrations measured in our study were comparable with results from studies on trace metal contents of vegetables grown in the vicinity of smelters or irrigated by wastewater. This clearly underlines potential health risks associated with urban horticulture in inner city areas [emphasis added].”
In contrast, this study regarding nuts and fruits harvested in Berlin found that heavy metal content of fruit was generally lower than that of vegetables, and mostly fell below EU standards. They ultimately concluded that “the consumption of urban fruits is not harmful to human health and fruit trees and shrubs can be considered more suitable for urban gardening in highly polluted areas compared to vegetables.” As with the urban-grown vegetable study, they found that barriers such as hedges and walls minimize contamination effects and that higher traffic densities result in higher contamination levels. They also determined that nuts generally absorb the fewest heavy metals, followed by pome and stone fruits (apples, pears, peaches, plums, etc.), followed by berries.
This article focuses on avoiding contaminated soils, but of course, water can also become polluted and unsafe. When harvesting aquatic plants, take steps to prevent contracting Giardia, an intestinal parasite that causes cramps, vomiting, and watery diarrhea. This microscopic parasite is passed through feces, and enters urban waterways through sewage and animals defecating in or near the water. The good news is that Giardia is destroyed by cooking, so it's fairly easy to avoid.
Urban waterways also become polluted by runoff from streets, golf courses, and industrial sites. Certain aquatic plants absorb heavy metals from water, with roots absorbing the most, then stems, then leaves. Notably, the Typhaceae family is on this list, which includes the common cattail Typha latifolia- a popular wild edible. It is probably best to refrain from eating the roots and rhizomes of aquatic plants in polluted areas.
I would be remiss if I didn’t mention the discrepancies between various studies. To quote the study about vegetable horticulture in Berlin: “the correlation between trace metal content in soils and trace metal content in crops cultivated in these soils is often poor or inconsistent. Non-toxic concentrations of trace metals have been found in vegetables grown in contaminated soils. In contrast, another study found toxic levels of trace metals in vegetables cultivated in uncontaminated soils.”
Furthermore, a recent foraging study conducted in the Bay Area found that, “even grown in soils with elevated levels of heavy metals, tested species were safe to eat after rinsing in cold water.” Additionally, sampled species had no detectable levels of multi-pesticide residues, PCBs, and glyphosate. However, the sample size was small— nine samples for the heavy metals and six for the pesticides. Therefore, it is not prudent to assume that all plants growing in risky areas are safe for consumption.
To be sure, more research is needed. To quote this foraging study from the Johns Hopkins Center for a Livable Future in Baltimore, “While recent studies have used quantitative and geospatial methods to examine foraging in African cities, to our knowledge, ours is the first to do so in North America… foraging remains largely unrecognized in urban policy, planning, and design, except where prohibited by regulations governing public parks and other greenspaces. A clearer understanding of the potential health benefits and risks associated with foraging could inform how these practices might explicitly be incorporated into urban food system policy.”
So, to some extent, it’s pick at your own risk. But like I said earlier, don’t panic! As evidenced by the recent E.coli outbreak in romaine lettuce, even food bought at the grocery store is not risk-free. While we may long for food that’s completely pure— free of heavy metals, dangerous chemicals, and illness-causing microorganisms— that’s simply not possible in a world inundated with those things. As stated earlier, the key is to minimize exposure. And with foraging, you have the benefit of direct experience with the food you will eat; you can see what condition it’s growing in, and decide whether to harvest it or not.
Furthermore, as recognized by the Johns Hopkins study, foraging has other benefits. “The practices of seeking, harvesting, preparing, and consuming forageables sustain the cultures and identities of diverse urban populations, including indigenous peoples, settler populations, and recent immigrants. At a time when over half the world’s population lives in cities and the pace of urbanization is increasing, foraging creates connections to nature and biodiversity. Foraging can be a no-cost source of fresh, micro-nutrient dense food, which may be particularly beneficial for low-income and/or food insecure households. The importance of urban foraged foods has been demonstrated in times of collective crisis such as armed conflict or natural disasters. In contrast to urban agriculture, foraging entails virtually no entry costs beyond the knowledge and time required for harvest. Urban forests may harbor large stocks of species with resource benefits for city residents, albeit with some species of interest to foragers found in low abundances. Yet foragers may engage in stewardship practices that minimize their harvests of certain species and reduce their impacts on these systems. These characteristics suggest foraging could contribute to healthy, diverse, and resilient urban food systems.” For me, this makes the practice of foraging not only worthwhile, but vital to our collective future. I look forward to a day when the human health effects of urban foraging are better understood and the practice proliferates in our cities.
I understand that this is a lot of information to absorb! Here’s the take-away:
There have been few studies that focus on urban foraging, but we can use preliminary results and those of agricultural studies to establish guidelines.
The single most important practice may be to wash what you harvest before you eat it! Most contaminants are ingested from dust or soil that’s blown onto the plant, rather than from the plant material itself.
Beware of potentially contaminated areas, such as roadsides, industrial sites, railroad tracks, and golf courses. The farther you forage from a hazardous site, the better. Barriers such as buildings, walls, and hedges reduce the overall contamination level. Lower traffic densities are typically safer than higher ones.
In general, nuts absorb the fewest heavy metals, followed by pome and stone fruits (apples, pears, peaches, plums, etc.), followed by berries, followed by vegetable crops.
When harvesting aquatic species, cook what you harvest before eating to avoid contracting Giardia, an intestinal parasite that causes cramps, vomiting, and watery diarrhea.
It is probably best to refrain from eating the roots and rhizomes of aquatic plants in polluted areas.
Foraging is a vital activity! Have fun!
Note on Sources
The sources cited above come from scholarly articles and public documents. Some are free, while others are behind a paywall. Here are links to the free original documents:
These documents are behind a paywall:
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