Theory of Earth’s free oscillation is revisited. Firstly, we summarized the underlying formulations, such as the equation of motion and its conversion into numerically integrable form and then explained computational procedures including the treatment of inner core-outer core boundary and core-mantle boundary, while the latter information has not been explicitly given in most publications. Secondly, we re-calculated the periods of Earth’s free oscillation modes (period >200 s) for PREM model. In doing so we acquired the values of modes missing in Dziewonski and Anderson (1981). As a case observation, one seismogram after 2011 Tohoku earthquake recorded at Daejeon, Korea (KIGAM seismic station) was briefly analyzed to identify free oscillation mode excitations on its spectra. The material in this article will be most clear guide for those on calculating the Earth’s free oscillation mode.

In this study, in order to analyze the air quality of the indoor environments of schools, we measured the indoor, outdoor and personal exposure concentration level of PM10 for 40 classrooms(20 old, 20 new) in chungnam area from June 22 to July 19 and from November 21 to December 30, 2003. 1. Old classrooms contained more dust than new classrooms; the average of respirable dust is 43.27 ㎍/㎥ for new classrooms while 53.38 ㎍/㎥ for old one. The exposure concentration level of respirable dust in new classrooms were in summer higher outdoors than indoors. The values were indoors 46.71 ㎍/㎥, outdoors 50.46 ㎍/㎥, and personal 41.62 ㎍/㎥. Meanwhile in winter indoors had a higher concentration level than outdoors, the values being indoors 39.11 ㎍/㎥, outdoors 34.86 ㎍/㎥, and personal 49.01 ㎍/㎥. 2. Cr concentration level within dust was slightly higher in summer indoors (101.50±32.10 ng/㎥) and outdoors (100.89±35.18 ng/㎥) than winter indoors (85.80±48.95 ng/㎥) and outdoors (74.43±38.93 ng/㎥), but in personal concentration level, winter was higher. The results of this research show insufficient understanding of health risks from indoor air pollution, and shows possible health problems to students from school indoor air pollution. As such, a logical and systematic education program for students about the importance of indoor air quality should be carried out. Also the results of PM10 concentration level measurements emphasize the need for regular measurements of indoor / outdoor and personal concentration level. New classrooms in particular needs to be used after measuring pollutants and safety, and requires installation of a ventilation device in all classrooms to improve air quality.