Ultrafine Particle Concentrations and Exposures in California Classrooms

WILLIAM W NAZAROFF (1), Nasim A. Mullen (1), Seema Bhangar (1), Nathan M. Kreisberg (2), Susanne V. Hering (2)

(1) University of California, Berkeley, (2) Aerosol Dynamics, Inc.

     Abstract Number: 197
     Last modified: November 8, 2009

Abstract
Laboratory studies, epidemiological evidence, and atmospheric monitoring studies during the past two decades have combined to raise significant concerns about the potential health risks that may result from environmental exposure to ultrafine particles (UFP), i.e. those smaller than 100 nm in diameter. However, our collective understanding of the human exposure to ultrafine particles is limited. Most environmental monitoring has occurred outdoors, yet people spend a large majority of their time indoors. One setting that has received very little attention and is of particular concern is classrooms in urban public elementary schools. To help bridge the gap, we made time-resolved (1-min resolution) continuous measurements of particle number (PN) concentrations over a cumulative period of 18 days in six classrooms during normal occupancy and use. These classrooms were located in the East Bay area, near San Francisco, California. Particle monitoring was done indoors and outdoors with a pair of water-based condensation particle counters with a minimum measurement size of 6 nm. Carbon dioxide levels were also continuously monitored to characterize the ventilation conditions of the indoor spaces. On each monitored day, while school was in session, a researcher also was present in the classroom to record with one-minute resolution the occupancy level, the position of doors and windows, and indoor source-related activities.

During times when classrooms were occupied by students, we found that the average indoor PN level ranged from 5,200 per cubic cm (at site S5) to 16,500 per cubic cm (at S1) with an overall average across all six sites of 10,800 per cubic cm. Outdoor average concentrations during occupied periods varied from 9,000 per cubic cm (S5) to 26,000 per cubic cm (S1) with an overall average of 18,100 per cubic cm. The ratio of average indoor to average outdoor concentrations (I/O) during times of occupancy varied from 0.48 (S2) to 0.77 (S3) with an overall average of 0.59. Both outdoor and indoor levels, as well as the indoor/outdoor ratio tended to be higher during periods of occupancy than when classrooms were vacant. When classrooms were closed and mechanical ventilation fans were off, the average air-exchange rates (AER) were 0.3-1.0 per hour and the I/O was 0.19-0.52. However, most of the time that the room was occupied, either windows or doors were open or the mechanical ventilation system was on, and under these conditions the average AER was in the range 2.1-4.6 per hour and the I/O level varied between 0.42 and 0.72. At these school sites, indoor PN was predominantly of outdoor origin. The average daily exposure for students in the monitored classrooms varied from 11,000 h per cubic cm (at S6 on 8 December 2008) to 100,000 h per cubic cm (at S1 on 6 June 2008), with an overall average for the 18 monitored days of 52,000 h per cubic cm. The daily average classroom PN exposure of students at the six sites was well correlated with the average outdoor PN concentration during occupancy (r squared = 0.9).