"Under the Clean Air Act, EPA is required to periodically review its clean air standards - those for carbon monoxide, lead, nitrogen dioxide, sulphur dioxide, ozone, and particulate matter - and change them if scientific studies warrant. In revising and setting standards, known formally as National Ambient Air Quality Standards (NAAQS), the EPA administrator must establish them with an "adequate margin of safety" to protect public health. The ozone standard was originally set in 1971 for a group of pollutants called "photochemical oxidants" (later changed to just ozone), not to exceed an hourly average level of 80 parts per billion, or PPB more than one hour per year. The ozone standard at the time was based, in part, on studies that showed increased asthma attacks in areas where ozone levels were high. By 1979, the standard was formally changed to adopt ozone as the indicator rather than photochemical oxidants, and the level was relaxed to 120 PPB ozone. EPA arrived at the revised standard after finding adverse health effects for sensitive individuals in the 150-to-250 PPB range, setting the standard at 120 PPB, providing what it then believed to be an adequate margin of safety. EPA also changed the violation threshold from a one-hour exceedance to a one-day exceedance. Compliance with the national standard for ozone - meaning whether a particular city or county had attained the standard-was determined over a three-year period."
This paper studies the relationship between violence in the Northern Triangle and child migration to the United States. It finds that one additional homicide per year in the region, sustained over the six-year period of study—that is, a cumulative total of six additional homicides—caused a cumulative total of additional unaccompanied child apprehensions in the United States. The explanatory power of short-term increases in violence is roughly equal to the explanatory power of long-term economic characteristics like average income and poverty.