Cities around the world are undergoing rapid urbanization, resulting in the growth of informal settlements or slums. These informal settlements lack basic services, including sanitation, and are associated with joblessness, low-income levels, and insecurity. Families living in such settlements may turn to a variety of strategies to improve their livelihoods and household food security, including urban agriculture. However, given the lack of formal sanitation services in most of these informal settlements, residents are frequently exposed to a number of environmental risks, including biological and chemical contaminants. In the Kibera slums of Nairobi, Kenya, households practice a form of urban agriculture called sack gardening, or vertical gardening, where plants such as kale and Swiss chard are planted into large sacks filled with soil. Given the nature of farming in slum environments, farmers and consumers of this produce in Kibera are potentially exposed to a variety of environmental contaminants due to the lack of formal sanitation systems. Our research demonstrates that perceived and actual environmental risks, in terms of contamination of food crops from sack gardening, are not the same. Farmers perceived exposure to biological contaminants to be the greatest risk to their food crops, but we found that heavy metal contamination was also significant risk. By demonstrating this disconnect between risk perception and actual risk, we wish to inform debates about how to appropriately promote urban agriculture in informal settlements, and more generally about the trade-offs created by farming in urban spaces.