The revelation came after a joint investigation by public broadcaster RTS and three Swiss newspapers showed that a large number of Lonza employees had been affected by mercury poisoning between the 1920s and 1940s.
The shiny metal liquid, once commonly found in medical thermometers, attacks the nervous system and can result in severe, life-long disability.
Lonza told the Swiss media outlets, and later confirmed to AFP, that it had hired a historian to investigate the matter following the media probe.
The historian's research of archives from that time showed that up to 250 employees at the Lonza plant in the southern town of Visp had suffered from mercury poisoning, the company said in an email.
Lonza used mercury as a chemical catalyser in industrial products from 1917 until 2013.
RTS, along with Swiss papers Le Temps, Le Nouvelliste and Walliser Bote, discovered reports written by doctor Paul Burgener — considered a pioneer in the fight against mercury poisoning — describing the cases of eight Lonza employees he had followed for two decades.
They suffered from a range of ailments, including loss of appetite, insomnia, chronic physical and psychological fatigue, impotence, tremors, and depressive and suicidal tendencies, Le Temps reported.
An article about Burgener published in a medical journal in 1952, a year after his death, also described how he “for decades fought widespread resistance to the recognition of this work-related illness,” the paper said.
Lonza said only some of the cases detailed in the archives were described as serious, and it insisted its management at the time acted appropriately.
The company had provided health screening to employees, and took measures to limit their exposure to mercury, including having their work clothing washed on site, it said.
It also said its insurers had covered medical costs once ailments were determined to be work-related, adding that there was no indication any of the affected employees had died from their exposure.
The last two employees know to have been affected by mercury poisoning died at the ages of 87 and 92 in 2001 and 2002, it said.
The investigation by the Swiss media outlets also dug up reports showing the company and the authorities in the canton of Wallis were aware of the problem of mercury ground contamination in the area surrounding Lonza's plant well before the public found out a few years ago.
The mercury levels Lonza discharged into a nearby canal between 1930 and 1976 had been below the legal limits at the time, but the mercury had accumulated in the sediment and sludge, which people often gathered to use as fertiliser.
While the contamination of the canal was known since the 1970s, the ground contamination only became apparent in 2011, sparking a massive clean-up that began late last year.
But according to the reports published Saturday, the company and the cantonal authorities were aware of the problem also at least back to the 1970s.
The ground clean-up is expected to cost about 51 million Swiss francs (44 million euros), and Lonza has agreed to foot 47.5 million of the bill, the ATS news agency reported.