Urban gardens are rapidly spreading in cities because people want healthier, self-grown food. However, cities are often highly polluted by atmospheric pollutants. Soils used for plant growth may also contain heavy metals. Agronomists Pennisi et al. grew vegetables with and without soil and found that soilless vegetables contained 70% less chromium, 61% less copper, 45% less cadmium and 81% less nickel. They conclude that, unexpectedly, soil is the major pollution source of urban vegetables, not air.