PUSAN, SOUTH KOREA, JUNE 21 -- Oxygen masks at Choon Hae hospital here have a special use these days. When tear gas wafts through the windows, as it does most evenings lately, they are placed over the faces of the most seriously ill patients to protect them from the acrid fumes.

To staff members, who must make do with such home remedies as wet gauze over their mouths, the fumes in the hospital have become routine.

Since fierce antigovernment protests began in South Korea two weeks ago, police have blanketed whole sections of densely populated cities with tear gas day after day. After police disperse protesters with the gas, adults and children who have nothing to do with the demonstrations are left gagging in their homes and workplaces. The total number of people who have suffered from the tear gas may well be in the millions.

"It is hard on the youngsters," said a woman walking with her 9-year-old granddaughter on a gas-tainted street here this evening. "They shed tears. Their noses run. I hope it's not too bad for their health."

Ordinary citizens have shown some unusual sympathy for the recent protests. By many accounts, an important reason is anger over the often trigger-happy approach that the police take with gas.

Police think nothing of exploding gas alongside packed buses that are caught in traffic jams. They throw tear-gas grenades onto busy sidewalks, getting as many shoppers as demonstrators.

The government calls it a humane tool that helps avoid fatalities. "We view tear gas as a defensive tool necessary to head off violent demonstrations and minimize damage," the country's National Police Headquarters said in a recent statement.

Police sometimes fire hundreds of canisters into a single gathering. Carried aloft by the wind, the gas can be transported for many blocks and descend on completely peaceful sections of the city.

Tear gas is actually a very fine powder, which sticks to clothing and is taken home or to workplaces. Foreign journalists brought so many of the particles into a recent news conference by ruling party chairman Roh Tae Woo that he apologized for the discomfort it was causing.

Tear gas causes sharp burning in the nose and throat. It painfully attacks the eyes, making them shut involuntarily and pump out what seems like gallons of tears. Some people panic at being suddenly rendered blind.

A very strong dose will sting whatever skin it touches. Prolonged intense exposure can cause the skin to blister and flake in what looks like a radiation burn. News photographers who chase riots all day are among the very few people who get it in these doses.

So in many city areas, no sensible person leaves home these days without protection.

A 12-year-old girl who was out for a stroll with two friends here today carried a terry cloth surgical mask made by her mother. When wetted, it helped to filter out gas from the air.

Some people plug their noses with cotton or gauze. Dabs of toothpaste may be added to the plug or under the eyes, in the belief that it will capture some of the particles.

Cellophane plastered over the eyes offers some help. So do bandannas worn bandit-style over the nose and mouth. Swimming goggles protect the eyes and have become a hot item among street vendors.

A few sturdy souls resembling Buck Rogers space travelers can be seen striding along busy streets, their heads encased completely in inflated plastic bags.

Tear gas is the only real weapon the police use here. It comes in several forms. Gray grenades -- "apple bombs," they are called -- are thrown by hand, often at head level like rocks, giving people double reason to scatter.

Coffee-can sized canisters are fired high in the air by squads that stand in the rear of police lines using converted shotguns. They explode in midair, releasing their contents out in a white cloud over a wide area.

For serious protests, the police deploy a black armored car. From a hole in the roof, they fire a weapon that discharges dozens of canisters in seconds. The cloud it creates is as dense as a smokescreen.

Flying canisters occasionally cause serious injuries. Earlier this month a student was hit on the head by one and is now in a coma. He is expected to die.

That incident helped make the gas a political issue. Seoul newspapers have been calling on the police to show restraint. "Use must be strictly limited to unavoidable situations for the maintenance of law and order," the English-language Korea Times newspaper editorialized earlier this month.

Environmentalists say tear gas is killing grass and trees on university campuses. Other people suggest that it could cause serious health hazards over the long term. The police say the gas has been ruled safe by the World Health Organization.

The Korean National Council of Churches, a leader in the antigovernment fight, last week called a special anti-tear gas demonstration. People chanted slogans condemning the Korean companies that produce the gas.

To no one's surprise, the police broke it up with gas.

Many foreign journalists go to hardware stores in Seoul's Chung Gye Chon area where black police-issue gas masks can be bought for about $45. Merchants are not allowed to sell them to ordinary Koreans for fear that they will get into the hands of demonstrators.

Open defiance of the gas is one of the ways protesters show contempt for the authorities. While the police hide behind masks, students prance a few yards in front of their lines.

Their seeming immunity has set a few people here wondering, not totally facetiously, if the students are in fact addicted to tear gas and must have some every day.

If that is true, the students have little fear of running low. In 1986 the police set off more than 300,000 canisters, according to official figures. The figure this year is certain to be far higher.