Singapore is not affected by the haze throughout the year. Any exposure is short-term in nature (i.e. continuous exposure to unhealthy daily average PSI levels over a period of a few days) and such exposure may vary from year to year. As international studies are based on long term exposure to air pollution, there is little robust data on the longer-term effects of short-term exposure to haze like the pattern seen in Singapore. Studies have shown that persons living overseas with continuous exposure over several years to high ambient pollution from fine particles (i.e. particulate matter 2.5 (PM2.5); particles smaller than 2.5 micrometers), may have a higher risk of (i) cardiovascular effects, such as heart attacks, (ii) reduced lung development, as well as (iii) the development of chronic respiratory diseases, such as asthma, in children.