Why 25 feet?

Tam, There is VAST evidence that second hand smoke is a killer. The introduction of smoke into the buildings occupied spaces MUST be prevented at all costs (the litigation costs alone out-way the capital costs of smoking prevention within a building). The USGBC (in conjunction the ALL health advisers) have designated that smoking should be prevented closer than 25ft from the described areas. The only problem with this is that a city center building cannot control who smokes and where in a public street (unfortunately) but as attitudes changes, which they are...and legislation is further enhanced, this problem will go away. This rules INCLUDE the newest toy electric cigarettes.

As for 'why' I think it comes from separate ventilation in the codes and 25' was the strictest setting. We do healthcare work and the standards were to always separate an exhaust vent by 25' to an intake. It allows the pollutants to be dispersed. My assumption has been that this is where the standard was set. Notice the words assumption and were. Do I have an exact code citation? No, this is my interpretation. You might try digging around in the EPA website for air quality standards.