ENVIRONMENT officials are investigating unusually high levels of sulphur dioxide in Mongkok last month after readings on two days were eight times higher than the daily average. The abnormal readings, which occurred on April 23 and 30, have mystified the Environmental Protection Department. ''It is very unusual to have isolated high results like this and we still do not have a clear idea of what caused it,'' said principal environmental protection officer, Raymond Leung Pak-ming. Levels of sulphur dioxide pollution before and after both days were well within the department's air quality objectives. Mr Leung said the conditions posed ''no general threat to health''. Calm conditions last month caused an increase in sulphur dioxide and nitrogen oxide pollution at the department's three monitoring stations in Mongkok, Central/Western and Kwai Chung.