Police in the Homicide Unit said pepper spray is nonfatal. The safety of the chemical, however, has been called into question in the past. A 2003 study by the National Institute for Justice found that the spray is generally safe, but can potentially cause death in those with asthma or other respiratory conditions.
(From sept 2011)
Cops probing pepper-spray death (philly.com):
Homicide detectives are investigating how a 48-year-old man died in SEPTA police custody after officers sprayed him with pepper spray Friday night.
A sergeant in the Homicide Unit said SEPTA police responded to a call of a theft at the Wayne Junction rail yard, on Windrim Avenue near 18th Street in Logan, where they spotted Willie McCray, who had allegedly trespassed onto the property.
Police said McCray ran from the officers and hid in bushes nearby, then grabbed an officer when he was approached. A second officer then sprayed him, police said.
http://publicintelligence.net/philadelphia-man-dies-after-being-pepper-sprayed/