How healthy are urban horticultural crops? A study of the accumulation of heavy metals in vegetables grown in Tehran

Introduction:
Urbanization and population growth, in addition to its rapid development across the world, has caused a major demand for the food security and the self-sufficiency in terms of food production in many cities. According to the aforementioned circumstances, the activities in the basis of urban agriculture with different traditional methods (e.g. planting on the ground surface with a soil bedding) or the modern ones (e.g. rooftop gardening with hydroponic cultivation methods) have been emerged and numerous studies regarding these methods have been accomplished. Likewise, a large number of farms in the urban areas came into existence worldwide. Nonetheless, there are yet various challenges to develop this type of agriculture. Moreover, only few numbers of studies were carried out to monitor the health and hygiene condition of the food productions grown in such places, especially in terms of air pollutants accumulation. The objective of this research was to study the hygiene of the plants cultivated in urban environment (rooftops and courtyards), and to determine the accumulation rates of pollutants in the eggplant fruit (Solanum Melongena var. depressum (L.)) and bell peppers (Capsicum fruitescens var. grossum (Mill.)).
Materials and methods:
Two types of vegetables; bell pepper and eggplant were cultivated in the rooftop and courtyard of a five story building in the 7th district of Tehran. The seeds were planted on the basis of a completely randomized design system using six times repetitions. Besides, in order to compare the hygiene of these fruits with those of the fruits cultivated in the countryside, similar samples of the same products were used in the farms located in the environs of Varamin and Karaj. Densities of heavy metals such as molybdenum, chrome, copper and manganese ( Mn, Mo, Cr and Cu) included in the fruits of belle pepper and eggplants cultivated in the urban areas of Tehran were compares with their amounts in the fruits cultivated in the farms located outside of the towns. Additionally, the amounts of heavy metals Lead and Cadmium (Cd, Pb) in the samples related to the city of Tehran were compared to the European Union Standards and the Iranian National Standard. The sample digestion was carried out using Microwave, and in order to analyse and measure the results, the device of ICP-MS was used. Data was analysed by the SAS and Microsoft Excel software and the means comparison was done through standard error.
Results and discussion:
The resulted data demonstrate that the densities of heavy metals in the most of the fruits grown in city showed significantly higher numbers in the range of 5% than those of plants grown in countryside, except the contents of chrome in eggplant which is higher in the samples of the farms located in the countryside than those of both urban areas. Moreover, the heavy metal pollutants of copper in both eggplant and bell pepper from the farms located in countryside were higher than the samples of the rooftop. The chrome density in the eggplants cultivated in farmland was 1.56 times more than that of courtyard, and 1.58 times more than that of rooftop. Densities of copper in the eggplant and bell pepper of countryside farm are respectively 1.22 and 1.24 times more than the samples of rooftop. Furthermore, except lead contents (Pb) being observed at three different levels 0.18, 0.63 and 0.14 milligrams per one kilogram of dry weight (mg/kgDW) in some samples, respectively for rooftop eggplants, rooftop bell peppers and bell peppers of courtyard, which showed significant variances to the permissible limits (in the range of 5%), other samples indicated acceptable amounts of lead and cadmium with respect to the European Union Standard and Iranian National Standard (under the limits of 5%).
Conclusion:
In general it can be concluded that all the samples cultivated in the urban areas of 7th district of the city of Tehran were healthy in terms of allowable amounts of Cadmium as an heavy metal. These density ratios fully complied with international standards, and were similar to those of the samples cultivated in the countryside. In contrast, most of the aforementioned samples contained the impermissible amounts of lead heavy metal. Likewise, the densities of other heavy metals included in products of urban farms were much higher than those of the city exteriors. As a matter of fact, this difference might be resulted from the effects of heavy traffic flow, geographic position and the unsuitable climate conditions on the air quality of urbanized areas.