Sustainability.Agricultural production systems in urban and peri-urban areas can pose risks to public health and the environment. These arise from the inappropriate or excessive use of agricultural inputs - including pesticides, nitrogen, and raw organic matter containing heavy metal residues - which may leach or runoff into drinking water sources, microbial contamination of soil and water, and air pollution. In particular, leafy vegetables can be contaminated through overuse of chemical sprays, while zoonotic diseases and veterinary public health problems can arise from intensive livestock production.
The most viable source of water for urban and peri-urban agriculture is recycled treated wastewater. FAO has estimated that typical wastewater  effluent from domestic sources, when appropriately treated for agricultural reuse, could supply all of the nitrogen and much of the phosphorus and potassium that are normally required for agricultural crop production. Unprocessed liquid waste (e.g. pig slurry, flush waters) or semi-processed waste is sometimes used for fertilization, and raw chicken and cattle manure enhance soil fertility and structure. These practices carry some health risk, but when properly managed, this risk is minimized.
The major danger in utilizing waste waters is food contamination by pathogenic micro-organisms and outbreaks of water-borne diseases. High health risks associated with the use of untreated or improperly treated sewage water in irrigation include infection from helminths, while medium to low risk is associated with enteric bacteria and viruses. In general, evidence suggests that negative health effects are a problem only when raw or poorly treated wastewater is used for irrigation.
Another water quality issue arises in intensive aquaculture in peri-urban areas. Intensification implies heavier use of water for recirculation, commercial feed and drugs (antibiotics, bacteriostatics). Excess nutrients and organic matter enhance the proliferation of micro-organisms that lead to eutrophication by depleting dissolved oxygen in the water systems.
Another major challenge to the viability of UPA is land availability. Looming over many urban farmers, both men and women, is the constant threat of losing access to their plot and being forced to stop production. In many areas, non-farming households' inability to access land in the city is the major reason given for not farming. However, one of the paradoxes of UPA is that substantial agricultural investments appear to occur on very insecure holdings. Good markets for agricultural produce guarantee high short-term returns and make the risks of agricultural investment worthwhile, even on insecure lands.
Easing ecological problems
As urban areas grow in population, they expand outward, often overwhelming the natural environment, destroying ecosystems, and drawing resources from well beyond their defined limits. Cities' dependence on massive and relentless imports of food, energy, and other resources from distant areas, and often on exports of their wastes to those areas, can also be destructive. The city's ecological footprint has long been a problem in the cities of the North (Rees 1997). Now, the rapid and usually unplanned growth of many cities in developing countries, coupled with rising consumption levels, is also putting a strain on the natural resource base of the South.
Urban agriculture alone will not solve the ecological problems of growing cities, but it does help to protect the environment in a variety of ways. For example, in many cities urban farmers make productive use of many organic waste products, turning them into soil-enhancing mulch. Wastewater  can be used to irrigate crops. By cultivating every available piece of open space — even rooftops — urban farmers contribute to the greening of the city, helping to reduce pollution and improve air quality. Even the fact that less food has to be trucked in to the city contributes to sustainability and has a positive environmental impact. Urban agriculture helps to reduce a city's ecological footprint even as the city continues to grow (Nelson 1996).
Coupled with an improved environment is an overall improvement in the health of the urban population. A more livable city is a healthier city. For the urban poor in particular, the availability of fresh vegetables and other foods coupled with increased opportunities for income means improved overall health, and perhaps the opportunity to break out of the cycle of poverty.
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