The concept of causal webs, which emphasises complex multiple causation of disease and injury, diverts attention and resources from preventive brooms that can sweep away the webs to reduce harm despite multiple causation. Epidemiologists who focus primarily on specification of causal paths in webs may overlook the modifiable necessary conditions for disease and injury. Necessary causes for disease and injury and necessary conditions for prevention are not always the same. Corporations and governments have the opportunity and means to reduce or eliminate many hazards to human health by changing products and processes that are hazardous. The arguments used to avoid doing so are often flawed and sometimes false. Public health scientists who focus on the behavior of corporations and government agencies that fail to employ preventive brooms of known effectiveness are more likely to contribute to harm reduction than those who focus on causal webs.