Cotton masks with a manually added filter were as effective as government-certified masks in blocking particles of human saliva, research by a public health institute showed Friday.The average particle collection efficiency of cotton masks with an electrostatic filter cotton attached inside was found to be 80 to 95 percent, while government-certified KF 80 masks measured at over 80 percent, according to Seoul Research Institute of Public Health and Environment.Efficiency of dental masks came to 66 to 70 percent, while the efficiency of cotton masks without a filter dropped to 16 to 22 percent, it added.“It looks like plain cotton masks are effective when blocking large droplets (over 3 square micrometers). But for complete prevention it would be preferable to use them after adding electrostatic filter cotton inside,” Seoul Research Institute of Public Health and Environment said.Amid a shortage in supply and hike in prices, handmade filter-added masks that are washable and more affordable have become a practical option for many Koreans.The public health institute said handmade masks had to be washed and the electrostatic filter cotton replaced after use in order to maintain their particle prevention capacity.By Kim Bo-gyung ( lisakim425@heraldcorp.com