South Korean prosecutors on Friday questioned the former heads of major discount chains Lotte Mart and Homeplus as part of their high-profile probe into the toxic humidifier disinfectant scandal.

The Seoul Central District Prosecutors' Office summoned Lee Chul-woo, former chief executive of Lotte Mart, and Lee Seung-han, former president of Homeplus. Both led the companies when the retailers sold allegedly toxic humidifier disinfectants between 2004 and 2011.

In 2012, some victims of the hazardous humidifier disinfectants and their supporters filed a complaint with the prosecution against the pair, arguing that they should be charged with involuntary manslaughter.

"My heart really aches because of this case and I sincerely apologize to the victims and their families," Lee Seung-han told reporters as he appeared at the prosecution office in southern Seoul in the morning.

Lee Chul-woo appeared before the prosecution in the afternoon, saying that it was "very regrettable" that the scandal broke out.

"What shouldn’t have happened seems to have occurred. I feel very sorry about it," he told reporters.

Prosecutors are looking into whether the former chiefs of the retailers engaged in the process of developing the products in question that caused dozens of deaths.

Lotte Mart and Homeplus are alleged to have sold humidifier disinfectants containing polyhexamethylene guanidine, a hazardous chemical, without conducting any thorough testing on the potential toxicity of the products.

The prosecution is expected to wrap up its investigation and determine against whom they will file criminal charges as early as next week.

The government has found that a total of 74 people suffered lung problems after using the humidifier disinfectants produced by the two retailers. Of them, 28 are thought to have died due to the toxic chemical in the products.

The humidifier disinfectant case, one of the worst scandals involving a consumer product using chemicals, came to light after four pregnant women died of lung problems from unknown causes in 2011.

A government-led investigation confirmed a connection between the people who died of lung problems and the chemicals in products used to clean household humidifiers. (Yonhap)