An impressively large number of politicians, opinion makers, scholars, bureaucrats, and ordinary people hold that terrorism -- and al Qaeda in particular -- poses an existential threat to the United States. This alarming characterization, which was commonly employed by members of the George W. Bush administration, has also been used by some Obama advisers, including the counterterrorism specialist Bruce Riedel. Some officials, such as former U.S. Secretary of Homeland Security Michael Chertoff, have parsed the concept further, declaring the struggle against terrorism to be a "significant existential" one.
Over the last several decades, academics, policymakers, and regulators worldwide have developed risk-assessment techniques to evaluate hazards to human life, such as pesticide use, pollution, and nuclear power plants. In the process, they have reached a substantial consensus about which risks are acceptable and which are unacceptable. When these techniques are applied to terrorism, it becomes clear that terrorism is far from an existential threat. Instead, it presents an acceptable risk, one so low that spending to further reduce its likelihood or consequences is scarcely justified.
An unacceptable risk is often called de manifestis, meaning of obvious or evident concern -- a risk so high that no "reasonable person" would deem it acceptable. A widely cited de manifestis risk assessment comes from a 1980 United States Supreme Court decision regarding workers' risk from inhaling gasoline vapors. It concluded that an annual fatality risk -- the chance per year that a worker would die of inhalation -- of 1 in 40,000 is unacceptable. This is in line with standard practice in the regulatory world. Typically, risks considered unacceptable are those found likely to kill more than 1 in 10,000 or 1 in 100,000 per year.
At the other end of the spectrum are risks that are considered acceptable, and there is a fair degree of agreement about that area of risk as well. For example, after extensive research and public consultation, the United States Nuclear Regulatory Commission decided in 1986 that the fatality risk posed by accidents at nuclear power plants