In a telephone interview, Duque said DENR tests would allow authorities to determine if residents could safely return to their homes.
"But while the results are not yet out, it is better for them to stay at the evacuation centers," he said. "This is for their own safety."
Local authorities have sent home hundreds of families from upstream Barangays Prenza II and Sta. Rosa I after the chemical flowed down the Marilao River towards Barangays Tabing Ilog, Saog and Lambakin downstream.
Anticipating fumes from the chemical, local authorities have set up two new evacuation centers to take in residents from these three barangays who will flee their homes.
The unknown substance was dumped in an irrigation canal in Barangay Prenza II Tuesday, seeping into the Marilao River, and has since been flowing downstream.
Marilao Mayor Efipanio Guillermo vowed to place behind bars those responsible for the dumping of the toxic chemical in the irrigation canal in a rice field in Barangay Sta. Rosa I.
"We will spare no one, dapat pagdusahan nila ang kanilang ginawa," he said.
Dr. Hjordis Marushka Celis, Rogaciano Mercado Memorial Medical Hospital director, where 13 people, including seven children are confined, said 66 residents of Barangays Prenza II and Sta. Rosa I were taken to the hospital on Tuesday due to difficulty breathing after inhaling the noxious fumes.
Dr. Joy Gomez, the provincial health officer in-charge of public health, briefed Duque on the help given by health officers to victims.
When Duque asked what kind of substance was dumped in Marilao, Gomez said it might take the DENR and other health experts at least seven days to determine the oil based substance based on water samples which they have taken from the scene.
Dr. Cecilia Puellas, Marilao municipal health officer, said some patients were confined at the hospital for longer periods for observation.
In Marilao three evacuation centers were set up in Barangays Tabing Ilog, Saog, and Lambakin, all located downstream of the Marilao River.
Arnold Vytingco, Municipal Disaster Coordinating Office head, said in a telephone interview that 40 families were taken to an evacuation center in Barangay Tabing Ilog, 46 in Saog and 105 in the Lambakin evacuation center.
Residents are being given first aid, and that those strong enough were sent home after local officials assured that there are no more fumes billowing near their homes.
Meanwhile, the Provincial Environment and Natural Resources Office in Malolos City called on residents of Marilao, Meycauayan and other nearby areas to avoid contact with water from the Marilao River.
Nelson Gorospe, PENRO head, said that the water is not safe but allayed fears that contamination will affect more barangays located downstream.
"The DENR have sought the help of Inchem Environmental Inc., an accredited waste treater to conduct cleanup and treatment operations using oil spill absorbent pads," he said.
They have placed floating barriers on the river to prevent the substance from flowing further downstream, he added.
Gorospe said the Environmental Management Bureau (EMB) will conduct ambient air monitoring for hydrogen sulfide in the affected barangays.
However, that would only be possible if they can borrow equipment from the EMB office in Western Visayas used in the Guimaras oil spill, he added.
Meanwhile, the DENR issued yesterday a notice of violation against the owner of the 10-wheeler-truck that dumped an estimated 4,000 liters of chemical wastes in an irrigation canal in Barangay Prenza II on Tuesday.
Lormelyn Claudio, Environmental Management Bureau director for Central Luzon, said aside from truck owner Evelyn Sto. Tomas, the owner of the property where the mixed used oil was dumped is likely to be issued a notice of violation.
The property owner could be held liable under Republic Act 6969, the Toxic Substances and Hazardous and Nuclear Wastes Control Act for plain negligence, she added.
Claudio said the chemical is threatening the fish pens and aquaculture in Meycauayan and Obando, also in Bulacan.
The substance has already reached as far as the Tabing Ilog Bridge at around 2 p.m. yesterday, and that the slick is now visible in the river, she added.
Claudio said the results of laboratory tests on water and air samples from site might come out in the next two to three days.
The water and ambient air samples were brought to the CRL Laboratory in Clark Field, Pampanga for examination, she added. – With Katherine Adraneda, Jose Rodel Clapano