Published on 11 December 2018
Sur le toit d’AgroParisTech. AgroParisTech

Nastaran Manouchehri, research engineer at the Paris Institute of Technology for Life, Food and Environmental Sciences (AgroParistech), Baptiste Grard and Philippe Cambier, researchers at the laboratory of Functional Ecology and Exotoxicology of Agroecosystems (ECOSYS/AgroParisTech/INRA), Christine Aubry, leader of the urban agriculture research team at AgroParisTech, and Emeline Becq, corporate social responsibility (CSR) and quality manager for RATP Real Estate, examine the issue of the healthiness of urban produce.

Urban agriculture projects are on the rise in France. In Île-de-France, for instance, they cover 367 hectares. Paris’ city council took up the topic and made it a political promise: by 2020, Paris should feature 100 hectares of green roofs and walls, 30 hectares of which are to be productive. The Parisculteurs calls for projects should help in that regard.

The visibility of these projects goes hand in hand with a growing awareness of the nutritional value and risks of produce grown in cities, where pollution is a constant concern.

Is it possible to cultivate healthy crops in urban areas? That is the question tackled in a series of research initiatives launched in 2012 as part of the T4P research project and conducted by a team of researchers from AgroParisTech and France’s National Institute for Agricultural Research (INRA), as well as urban agriculture project leaders. Research focused particularly on the healthiness of produce cultivated in rooftop gardens.

Sources of contamination in cities

There are two main ways in which urban crops can be contaminated.

First, through the earth, called soil or substrate, in which the crops grow. This can happen through a soil-to-plant transfer via plant roots. Contamination depends on the kind of vegetable, the soil properties and the nature of the contaminant. Leafy greens (lettuce, cabbages, spinach, etc.) and root vegetables (carrots, radishes, etc.) are thus more likely to be affected by pollution than fruiting vegetables (tomatoes, peppers, etc.).

Regarding contaminants, it should be noted that lead, for instance, is a less mobile substance than cadmium, and its transfer from soil to plants is more likely when the soil is acidic and contains little organic matter. The total concentration of a contaminant in the soil does not necessarily imply a risk of contamination, as other parameters must come into play for the contaminant to migrate towards the crop.

Secondly, contamination can occur through atmospheric deposits and the absorption of contaminants by the parts of plants exposed to the air. Leafy greens such as lettuce are more sensitive to air pollution than other vegetables due to their large surface area, which gives them heightened exposure.

There is also a direct way, by which some categories of users (urban farmers, garden visitors) ingest earth as it is laboured. Their work can expose them to significant levels of contamination when the soil is rich in contaminants.

Finally, it should be noted that health risks regarding the presence of contaminants in urban agriculture do not only depend on the contaminant levels in the produce or soil, users’ level of exposure through the ingestion of contaminated vegetables or soil also counts.

Urban contaminants and health risks

As far as atmospheric contaminants are concerned, we are especially interested in two types. These are worrying because of their potential effect on human health, whether they are carcinogenic or cause the dysfunction of a particular organ.

First of all are metallic trace elements (MTEs), often studied in urban agriculture because they are in the air, soil and water of cities. In the air, they are contained in particles emitted through human activities, particularly industry and road traffic.

Since 2000, these emissions have greatly diminished due to new legislation. In 2017, the air concentration of the MTEs most damaging to human health - cadmium, lead, arsenic and nickel - was mostly below the new thresholds.

The other category of contaminants that we have studied, which are rarely monitored in urban agriculture, are polycyclic aromatic hydrocarbons (PAHs). These organic contaminants can be released into the air following the incomplete burning of various materials. Their main sources are wood burning and road traffic.

These molecules are classified by various health authorities according to how harmful they are to human health. The Benzo(a)pyrene molecule (BaP), for instance, has a proven carcinogenic effect. Three other PAHs are considered possibly carcinogenic.

For MTEs, regarding fruit and vegetables, European regulations impose maximal thresholds for lead and cadmium only. The others, such as mercury and arsenic, are regulated in food products.

Regarding PAHs, the most dangerous (BaP and the total of the four most dangerous molecules known as ‘PAH4’) are covered by European regulations for transformed food products such as oils, dried herbs, baby food and baby milk. The lowest thresholds concern baby food. There is currently no legal threshold for PAH levels in fruit and vegetables.

Green roofs in Paris

As part of our study, we chose to examine rooftop gardens, a form of urban agriculture that is developing rapidly.

These roofs are used to cultivate fruit and vegetables, herbs and edible flowers for local consumption. Many businesses and associations such as Topager, Cultures en ville and Veni Verdi have installed such roofs in Paris.

Rooftop gardens are mainly affected by air pollution. The previously mentioned soil‑to‑plant transfer is only marginal, as rooftop gardens use substrates of known origin and quality. The earth is made up of materials added by gardeners such as green-waste compost, wood chips and coffee grounds. These substrates contain only tiny amounts of contaminants as they must meet quality standards such as the NF U44-551 standard pertaining to growing media.

For the same reasons, the ingestion of soil by farmers or gardeners is theoretically not a major source of exposure, unlike for in-ground or greenhouse models of urban agriculture, in which the earth may be contaminated.

Ten experimental vegetable gardens

As part of our research, we studied ten vegetable gardens in the Paris region. Four of them are on the roofs of shopping centres (Porte de Versailles, Vélizy‑Villacoublay, La Défense and Levallois‑Perret), four others span the roofs of RATP (Paris’ transport network) sites; one experimental vegetable garden crowns the AgroParisTech building and, finally, one is on the roof of a Carrefour car park at Villiers‑en‑Bière.

Experiments were carried out in beds filled with substrate made from wood chips, mushroom compost – coffee grounds and the mycelium from oyster mushrooms – and green-waste compost.

The intensity of road traffic near these sites varies greatly, from 1,000 cars per day for the roof of AgroParisTech to 250,000 cars per day for the major roads by the other sites. Furthermore, the heights of the buildings vary from 3 to 20 metres.

Potagers expérimentaux.

Experimental vegetable gardens.

Chard, cabbages and peppers

Concentrations in the two regulated MTEs, lead and cadmium, as well as those relating to mercury, were measured for three types of vegetables (leafy, fruiting and root) in each experiment.

By analysing 30 to 45 samples per kind of washed vegetable – tomatoes, carrots, radishes, strawberries, cabbages, chard, peppers – for each of the sites, we noted that levels for the three MTEs were, on average, between three to five times lower than legal European thresholds.

In the same way, BaP and PAH4 levels were measured for the leafy greens on the RATP and AgroParisTech roofs. The 45 samples of washed vegetables – different varieties of lettuce and chard – revealed concentrations in the most dangerous PAHs that were below quantification limits, but also below the lowest legal thresholds set by the European Commission – those for the preparation of baby food.

These results show the effect of washing and peeling vegetables on contaminant levels.

We also noted the effect of the site (existence of physical barriers and height) on contaminant levels. The further the gardens were from the ground, the lower the level. The existence of physical barriers and their position in relation to the source of contamination could also reduce contaminant levels.

These research results are being evaluated, except those for the AgroParisTech rooftop garden, some of which have already been published.

Innovative ecosystems and social benefits

In Paris, in the precise conditions we have mentioned for the contaminants studied, traditional vegetables cultivated in rooftop gardens meet current regulations. Upon the condition, of course, that best consumption practices be observed: the vegetables must be washed and peeled.

However, with the emergence of new crops for which we have only very few data are grown, it is crucial to study their sensitivity to air pollution. An example is aromatic herbs, a constant in the projects of urban farmers, or hops, used to brew beer, a new Parisian speciality.

Also to consider are the services offered by these innovative ecosystems, such as retaining rain water and fostering biodiversity, as well as their social benefits (educational activities, spaces for city dwellers).

Finally, given the different means of contamination, it is essential to guarantee the quality of the substrates used in these agricultural endeavours to limit contamination via soil-to-plant transfer and soil ingestion, particularly for green waste, because it contains plants cultivated in urban areas. Contamination via soil-to-plant transfer and soil ingestion should be carefully considered when using polluted sites for in‑ground urban agriculture.

Our participatory research programme, REFUGE (in French, the acronym stands for “risks in urban farms: management and assessment”), offers a methodology for assessing and managing risks in urban micro‑farms to face the issue of soil pollution.


Stéphane Besançon, Anne-Cécile Daniel, François Nold, Laura Bessouat, Antoine Juvin and Anne Barbillion took part in writing this article.


The original version of this article was published in French on The Conversation.