Fire has a couple of purposes in Vietnam. There is, of course, the main utility that burning trash is a quick and easy way to clean up litter and leaves.
There is another reason why you see people starting fires in front of their houses. It’s called vàng mã or votive fire. They burn an offering to ancestors the same way that Catholics burn a candle at their church, and for the same reason.
According to a Wikipedia article: They figuratively offer “multi-storied houses, luxury cars, motorbikes, laptops, toilets, servants, passports, rice cookers, TVs, other valuable items and of course, tons and tons of ‘Benjamins‘ go up in flames on the sidewalks of Vietnam. Not real ones, but paper models made from bamboo, collectively locally called joss paper (paper offerings).
What does this signify?
They burn a form of joss paper resembling legal tender bank notes as burnt offerings to the deceased. It is essentially a way of showing respect to the dead that has become an important part of the culture.”
“The practice of ancestor worship is largely religious, stemming from the belief that the spirits of the deceased are still interested in the affairs of the world. The idea is that the more offerings one burns for their dead relatives, the better their afterlives will be.”
“They believe that spending so much will bring them prosperity, good luck and good health in return for their generosity. For others, this is a bit of a way to resolve their ancestor’s financial problems. The spirit money is burned as a repayment of debt the deceased had accrued during life and had never been able to pay back.”
“Another belief is that this money is given as a tribute to the God of Death in return for a short stay or to escape punishment. These are few of the many reasons joss paper is burned.”
What’s in that smoke?
Sometimes the smoke is from trash fires. In these cases, the smoke is usually just leaves and paper. However, small amounts of Styrofoam can be accidentally burned when burning trash. Styrofoam is a trademarked brand of polystyrene foam, commonly used as thermal insulation and water barrier. In Vietnam Styrofoam is more commonly used in cheap containers.
According to healthfully.com, burning Styrofoam releases toxic chemicals and smoke that can damage the nervous system and lungs. Burning a small amount of Styrofoam accidentally won’t harm you or the environment significantly.
There is much less smoke in Da Nang than in Thailand. Thailand has very bad air quality during the fall because of field burning. The farmers burn the rice paddies stubble after harvest.