Residents living in the vicinity of the Mayon volcano have been warned by government officials about the possibility of acid rain as well as the ill effects of toxic gases as the volcano calmed down following two major eruptions last week.

Environment Secretary Heherson Alvarez cautioned returning residents to the Mayon volcano amid reports that most of the 41,000 evacuees had started returning to their homes even if the volcano, which is located some 350km south of the capital, Manila, had spewed steam and other gases, including carbon dioxide, sulphur dioxide as well as hydrochloric acid, hydrofluoric acid and ash into the atmosphere.

According to Alvarez, residents will be in danger from huge amounts of volcanic gas such as carbon dioxide and carbon monoxide which contribute to global warming, while others like sulphur dioxide can destroy the ozone layer. He said volcanic gas could be harmful to humans, animals, plants, agricultural crops and property, and the heavy rains being experienced right now could trigger not only heavy mudflows but acid rain as well.

Unlike mudflows which affect only the areas close to the volcano, acid rain could have an impact over a larger area. Citing health records, Alvarez noted that volcanic gases severely damage vegetation and causes serious defects to animals, while acid rain corrodes utility lines, farm equipment, cars and other metal objects and may release heavy metals into drinking water.

Alvarez thus urged Mayon residents to be alert to exposure to these gases which can damage eyes and the mucous membrane along with the respiratory system, while excessive inhalation of carbon dioxide and monoxide could cause death.

Meanwhile, the Philippine Institute of Volcanology and Seismology (Phivolcs) warned that Mayon may be just be building up volcanic force for another major eruption. Phivolcs Director Raymundo Punongbayan said: "The sporadic lava discharge, high sulphur dioxide emission rate, occurrence of tremors and volcanic quakes and inflated state of the volcano's edifice indicate slow magma ascent, possibly related to post-eruption recharging.

"With the continuing restive state of the volcano, Alert Level 5 remains in effect, which means that explosive eruptions may still occur resulting in considerable risk to areas within the six-kilometre radius Permanent Danger Zone (PDZ)," he continued. Last Saturday, a huge chunk of the volcano's famous cone was eroded by the continuous volcanic activity.

Mayon, one of the world's most active volcanos, is a major tourist attraction. Last year, the volcano underwent a series of eruptions which displaced 80,000 residents of the province of Albay. Last June 23, the volcano erupted once more, sending some 41,000 residents fleeing to nearby evacuation centres. So far there have been no reported direct casualties from both eruptions.