"What occurred was routine office defogging — a scheduled sanitation activity conducted inside the DENR compound," the agency said.
"This procedure uses disinfectant mist, compliant with workplace hygiene standards. It is not used for crowd control," it added.
The DENR stressed that no chemical agents were directed at rally participants, saying the mist seen outside the building was a result of the sanitation activity.
Meanwhile, the Quezon City Police District (QCPD), in its official spot report, said the protest along Visayas Avenue concluded peacefully with no untoward incidents.
Southern Tagalog groups held simultaneous protests at the DENR and other government agencies in Quezon City on Sunday, a day before President Ferdinand Marcos Jr.'s State of the Nation Address (SONA).
The demonstration, dubbed the "Hagupit ng Timog Katagalugan" Caravan, was led by Bagong Alyansang Makabayan Timog Katagalugan to highlight environmental issues affecting the region.
"Community leaders shave their heads to express their indignation against destructive projects that leave the mountains in the region bald," the alliance said in a statement.
The group added that the symbolic act reflected its commitment to defend communities and the environment "against attacks by bourgeois capitalists and their imperialist puppeteers."
Among the protesters' demands were the rejection of the US-led PAX Silica Initiative, a halt to mining activities, and the protection of the ancestral domains and rights of Indigenous Peoples.
The caravan also proceeded to the central offices of the Department of Agriculture and the Department of Agrarian Reform, where demonstrators urged the government to address pressing agricultural concerns and provide greater support to Filipino farmers.