Best Denki, an electronic store, sold S$350,000 worth of air purifiers and related items in 4 days, a 300 percent increase. However, experts from the Institute of Environmental Science & Engineering at the Nanyang Technological University said that only some of these products, like High Efficiency Particulate air filters, would effectively remove small particles from air. Other kinds of air purifiers like ionizing purifiers gave off ozone which could cause symptoms similar to those caused by the haze.