Because of health concerns, persistence, and long-term environmental effects, the impact of pesticides on agriculture and public health has been the subject of considerable research. Organophosphorus pesticides exert their acute effects by inhibiting acetylcholinesterase in the nervous system with subsequent accumulation of toxic levels of acetylcholine. Herbicides have widely variable toxicity. In addition to acute toxicity from high exposures, there is concern over possible carcinogenicity as well as other long-term problems. Improper use of herbicides may damage crop plants, especially if too large a dose is used, or if spraying occurs during a time when the crop species is sensitive to the herbicide. There are also apprehensions about the toxicity of some herbicides, which may affect people using these chemicals during the course of their occupation. The use of herbicides and other pesticides carries risks to humans through exposure to these potentially toxic chemicals, and to ecosystems through direct toxicity caused to non-target species, and through changes in habitat. People exposed to pesticides had over a fourfold increased risk to Non-Hodgkin's lymphoma (NHL), neuroblastoma, child brain development defects, Parkinson's disease, prostate cancer, leukemia in children, male infertility and miscarriage.