Oxygen production by trees varies among cities based on differences in number of healthy trees, growth rates, and diameter distributions. Cities with mostly small trees would require more trees on average to offset the oxygen consumption of one person. Percent of the population’s oxygen consumption offset by urban forests varies depending on population density and total oxygen production. Cities with high human population density (e.g., Jersey City and New York) tend to have the lowest proportion of their oxygen consumption offset by their urban forest. A commonly cited statement is an acre of trees (100% tree canopy) can provide enough oxygen for 18 people (e.g., American Forests 2006; Tree People 2006), but based on this study, this estimate appears to be high by at least a factor of two. The number is more on the order of eight people per acre of tree cover (100% tree canopy). Oxygen production per acre of tree cover will vary based on tree density, diameter distribution, and tree health and growth.