This study investigated how the public awareness of air quality affects people’s decisions to participate in outdoor activities. Given that the keyword search volume of particulate matter (PM) in Seoul, South Korea started to grow dramatically only after November 2013, we defined two periods (low and high public awareness of PM) and conducted a series of comparative analyses to investigate the impact of public awareness of air quality on the relationship between PM level and people’s outdoor activities. In the low public awareness period, people’s outdoor activities measured by the number of daily subway passengers did not significantly vary over PM levels, even in the ’unhealthy’ range (80 < PM10
< = 150 µg/m3
). On the contrary, during the high awareness period, people’s activities were significantly affected by the PM level, even in the ’moderate’ range. Specifically, the perceived safety threshold of PM10
level that people use to decide for engaging outdoor activities has decreased from 120 to 70 µg/m3
. These results suggest that public awareness of air quality and its harmful ramifications on health is a key determinant of outdoor activities rather than PM10
concentration itself. Thus, this study alludes to a need for more timely and effective dissemination of air quality information to the public as much as for curbing anthropogenic emissions.
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