A health official on Thursday said that disinfectants used for cleaning humidifiers could be the cause of a mysterious lung disease that claimed lives of four pregnant women.
The Korea Centers for Disease Control and Prevention (KCDC) began investigating after 28 cases of lung disease with unknown causes were reported since 2004, many of them this year and involving pregnant women.
Four of the pregnant patients died earlier this year.
The agency said tests showed that a harmful substance in a steriliser -- used by all the patients in their humidifiers -- could seriously damage lung cells.
Tests showed that those who added the steriliser to their humidifiers were 47.3 times more likely to get lung damage than those who did not, KCDC said in a statement.
"The odds ratio of 47.3, clinically speaking, is exceptionally high and rare," Youn Seung-Ki, a senior KCDC official, told AFP on Thursday.
KCDC refused to identify the harmful substance in the steriliser, which it said was widely used in other goods like shampoo and cosmetics, saying it could not verify whether the substance was also harmful through skin contact.
"We still need more tests to reach a concrete final conclusion. But we have decided, as a precaution, to urge people against using humidifier steriliser and companies from selling the products," it said in a statement.
Major steriliser producers had agreed to suspend sales until the final result of the agency's probe comes out in about three months, Youn said.
"The patients added the steriliser whenever they replaced water in the humidifiers and used a whole bottle (of steriliser) a month on average," KCDC said.
South Korean households often use humidifiers during the long, cold and dry winter.
The agency said pregnant women spend more time resting at home, increasing their exposure to air from the humidifiers. Women also inhale about 30 percent more air after childbirth, meaning they could be more at risk at that time.
About 600,000 units of humidifier steriliser worth two billion won ($1.8 million) are sold every year in South Korea. Makers of the product say it helps remove bacteria and dirt from the water before it is released into the air.