A contaminated air exposure level inferring apparatus may include a receiver for receiving a fine dust concentration measured from a fine dust sensor of the wearable device or the portable air purifier, an exposure level classifier for inferring a contaminated air exposure level of a user holding the wearable device or the portable air purifier based on the measured fine dust concentration, and a communicator for communicating with a server. The server may include an artificial intelligence model learner for generating an artificial intelligence model that has learned data on the measured fine dust concentration through a deep neural network. According to the present disclosure, it is possible to infer the contaminated air exposure level of the user of the wearable device or the portable air purifier by using artificial intelligence (AI), a contaminated air exposure level inferring technology based on the artificial intelligence, and a 5G network.