There was a sharp rise in the number of days potentially dangerously high levels of ozone were found in Afula, Tel Aviv and Jerusalem in 2007, the Environmental Protection Ministry said Wednesday. According to the World Health Organization, ozone has a one- to five-percent chance of causing a stroke depending on levels and exposure. The WHO instituted tighter regulations for ozone in 2005 after evidence began to accrue pointing to its potentially harmful effect. The ministry estimated that the higher levels were due to favorable atmospheric conditions combined with higher use of gasoline powered machinery. The highest levels were found in Afula, which had experienced a 135.7% increase from 2006 to 2007, from 56 to 132 days of higher than normal readings. Jerusalem was second highest with a 95% increase, from 45 to 88 days, according to statistics gathered by the ministry. Tel Aviv also recorded a 45.7% increase with 70 days of high ozone levels in 2006 and 102 in 2007. In contrast, Rehovot and Beit Shemesh experienced 36% and 22.3% drops, respectively. The ministry explained that there was a difference between the "good" ozone found in the earth's atmosphere and ozone found in cities, which is a pollutant. Avi Moshel, deputy head of air quality control at the ministry, explained that ozone was not usually released directly from factories or cars, but rather was the result of a chemical reaction caused by unburned gas particles released from factories and cars interacting with nitrous oxide from burnt gas particles in the presence of sunlight. The most common times of year for the reaction to occur, he continued, were during seasonal changes and the summer. The WHO standard for proper levels of ozone is no more than 51 parts per billion.