The probe came after a 2011 scandal involving toxic humidifier disinfectants sold by Oxy Reckitt Benckiser rattled South Korea, leaving more than 100 people dead from lung problems.
Hong Ji-ho, former president of SK Chemicals Co., and seven people were arrested, and 26 others, including a government official, were indicted without detention, according to the Seoul Central District Prosecutors' Office.
Since late November, prosecutors have been looking into SK Chemicals and Aekyung Industrial Co. for allegedly manufacturing and selling humidifier sterilizers using toxic chemicals without informing consumers of possible health hazards.
E-Mart was also being probed for selling private brand (PB) products after buying humidifier disinfectants from Aekyung Industrial.
The chemicals under scrutiny were chloromethylisothiazolinone (CMIT) and methylisothiazolinone (MIT).
After prosecutors' 2016 probe, officials at Oxy, Lotte Mart and Homeplus Co. were indicted for manufacturing or selling humidifier disinfectants containing the hazardous chemical polyhexamethylene guanidine (PHMG).
The 34 suspects indicted after the reinvestigation were not subject to prosecutors' initial probe, as a correlation between CMIT and MIT and harm to health was not verified at that time.
Their hazardousness later came to light as more studies proving their toxicity were released.
Prosecutors' reinvestigation has gained traction since the environment ministry submitted related data and studies to the prosecution in November last year.
The humidifier disinfectant scandal, one of the country's worst ever involving chemical consumer products, came into the spotlight after four pregnant women died of lung problems in 2011.
As of Friday, more than 6,400 people were registered as victims of the humidifier sterilizer case, of whom about 1,420 have died. (Yonhap)