In 1998, the Education Bureau's recommended schools to curtail outdoor activities when the index reached 200, whereas leading healthcare advocates are urging that the level be revised to 100. The World Health Organization revised its air quality guideline levels of sulfur dioxide, nitrogen dioxide and ozone in 2006 in light of new scientific evidence. The WHO also introduced new measurement guidelines for very small particulates which are more dangerous to pulmonary function. At the '200' level, Hong Kong levels of SO2 (800μg/m3) and NO2 (1,130μg/m3) are 40 times and 5½ times WHO guidelines respectively; the equivalent for particulates (350μg/m3) is 7 times WHO guidelines.