Assad has been pretty restrained. Homs has a population density similar to NYC's - 40K to 60K per square mile. If Assad had been firing hundreds of arty rounds into Homs (with its block after block of apartment buildings) on a daily basis, the body count would have been in the hundreds per day, instead of the highly dubious (given the tendency of Sunnis to exaggerate when it suits their purposes) 60 recently. When Assad pere put down the Muslim Brotherhood revolt in Hama in the 80's, the daily body count was 300-900 people dead, with tanks and artillery demolishing whole sections of the city.
In order for chemical weapons to be effective is the correct dispersal pattern or dissemination, and the effectiveness of delivery. The best way to deliver chemical weapons is through spray tanks by low flying aircraft but the pilot might have to be wearing gear. Also weather plays a factor.Wind, temperature, humidity, precipitation, terrain contours, and surface cover influence the field behavior of vapors and aerosols.