Twenty more firms have declared that their products are free of raw materials from Asia Pulp and Paper (APP), which is under investigation over its connection to the haze.
This brings the number of firms that have signed the Singapore Environment Council's (SEC) declaration form to 38.
The latest firms to sign are those that sell wood-related products under the SEC's green label scheme.
The online declaration form was sent to 210 firms in the wood-related product trade last weekend, said the SEC and the Consumers Association of Singapore in a joint statement yesterday.
It was sent to 23 companies earlier this month, including major supermarket chain FairPrice and APP's exclusive distributor here, Universal Sovereign Trading. The move led several retail chains to pull APP products off their shelves.
In another development, about 2,800 members of the Singapore Manufacturing Federation (SMF) have also been sent an appeal by both the SEC and the federation to commit to responsible sourcing.
SMF secretary-general Lam Joon Khoi told The Straits Times yesterday that the SEC approached the federation two weeks ago with the idea. The appeals were sent out last week and responses are trickling in.
He said: "It's not something that we expect them to change overnight, but at least we have started the journey.
"We really need to tighten the screws and increase the pressure to encourage firms to buy from more sustainable companies.
"Some have said this will increase their costs. But my point to them is that we already pay a high price for the effects of the haze.
"We will keep reminding them, and hopefully we can spark a bigger movement."
The hope, said the SEC's head of eco-certification Kavickumar Muruganathan, is for firms under its green label scheme to buy only from sustainable sources so that the credibility of the label would be upheld. He added: "We also want to encourage retailers to source more sustainably in the long run, and consumers to be given the information they need to buy sustainable products if they choose to."
Last month, the National Environment Agency served APP a legal notice to supply information on its subsidiaries in Singapore and in Indonesia. Six other Indonesian firms have been sent a notice asking them to take measures to extinguish fires on their land, not to start new ones, and submit action plans on how they will prevent future fires. The sixth firm, PT Bumi Andalas Permai, was sent the notice yesterday.
Mr Wilson See, the owner of Kronotex Singapore, which sells wood flooring, signed the declaration form last weekend.
"For us, it was quite straightforward; we checked with our suppliers and we don't take supplies from any of the companies under investigation," he said, adding that all his products are from Germany.
The APP took out advertisements in The Sunday Times and Chinese daily Lianhe Zaobao on Sunday to tell consumers of its no-burning policy. "We do not burn our land. We will disengage any supplier involved in starting fires... We are working around the clock to tackle both fires and the cause of fires in the wider landscape," read the half-page advertisement.
It added: "We will work with the Government of Singapore, its agencies, and all those who, together, can help find solutions to our shared challenge."