Even prior to the final EPA gas drilling air rule, gas emitted virtually no soot, mercury, or lead, while coal and oil emit a lot of some or all of many of those pollutants. That makes gas cleaner than coal oil in many ways that affect human health.
But the leakage rate of methane from gas production does matter for climate impacts and has been put at very different levels by different studies. The EPA in 2013 has lowered its estimate of the leakage rate, but everybody agrees the gas leakage rate can and must be reduced. And so the EPA final rule that takes affect in 2014 is good news, because it slashes by 95% emissions of volatile organic compounds during gas production. It also cuts methane emissions. http://www.epa.gov/airquality/oilandgas/actions.html.
The rule also means that shale gas may emit less methane than other sources of natural gas. Here is why.
The rule requires green completions/reduced emission completions must be used at high pressure wells like shale gas wells. Virtually all shale gas wells will be green completed by January 1, 2015. Yet, the rule does not require green completions at low-pressure wells (87% of coal bed methane gas wells are low-pressure).
Shale gas development also typically has newer gathering systems and newer equipment generally, because it is the most recent. Newer pipelines and equipment reduce methane leakage, compared to older systems that serve typically conventional gas fields.