Hands over their ears, frustrated students trudged out of fourth-period classes at approximately 10:38 a.m. today amidst a steady drizzle and earsplitting alarms in the year’s fourth non-drill application of the emergency evacuation plan.

The new alarm system, equipped with devices more sensitive to gaseous emissions than Paly’s former detectors, was activated by smoke from an advanced chemistry lab in room 1702 of the science building.

"I think there was an experiment in the chemistry room, … and a little fire started," assistant principal Doug Walker said, based on early reports about the cause of the alarm. Although the preliminary information was true, the connotations were somewhat misleading, since the fire was controlled and deliberate.

"We had the fume hoods on, but there were 20 Fischer burners on at once … so it could’ve been that," said senior Alex Gibson, who is in the fourth-period chemistry class.

Unlike previous instances, most students seemed to know the cause of the alarm; also, the time period between alarm and the 10:52 a.m. dismissal was relatively short, as the episode lasted about 10 minutes.

Initially, there seemed to be multiple explanations for the alarm, ranging from a smoke machine in the English building to an outdoor Special Friendship Club barbecue.

"We’ve been planning this barbecue forever, and I was so afraid we’d set [the alarms] off," said junior Alex Snell, who is the club’s co-president. Snell was relieved to learn the barbecue was not at fault.

Some now take a leisurely approach to the evacuation procedure, which has become almost routine for students.

"Normally I would love [evacuating to the football field], but it’s raining. It’s not as pleasant as it usually is," said junior Michael Benson.

Others, such as senior Eran Newman, are frustrated with the repeated disruptions. "This is stupid," he said. "It’s a waste of time."

Newman also expressed his hope for fewer of the alarms. "The chemistry rooms should get a better system so they’re not so sensitive to smoke, or else we’d be here every week."

Firefighters arrived at the scene to verify safety as the embattled chemistry students continued their labwork into lunchtime.

The preciptative lab, an intensive three-week-long final project that accounts for 25% of a student’s fourth-quarter grade, involves "smoking off filter paper burning in crucibles," according to another chemistry student, who wished to remain anonymous. "[The lab] takes a long time. … They’ll do it several more times," said the student, wryly adding that he would not be surprised if the situation should reoccur in the coming weeks.

The following reporters contributed to this article: Laura Mitchell and Emily Schickli.