Climatic impacts on human health can be direct or indirect. Direct impacts include variations in physical comfort, heat and cold stress, frostbite and – specifically in response to stratospheric ozone depletion – sunburn, sunstroke, skin cancer and (possibly) cataracts. Direct impacts also include death and injury from floods, storms and other extremes of weather. Photochemical air pollutant levels and pollen levels are affected by climate and have been related to asthma, other respiratory problems, and allergies. Through its influence on biological disease agents, climate variability has a major impact on infectious disease emergence and re-emergence, by affecting pathogen maturation and vector reproduction and altering host and vector habitats. Climate change is also predicted to alter regional agricultural yields, with downturns most likely in low-latitude countries where food insecurity often pre-exists.