Air has always been one of the few things in life that is free. But at some District gas stations, even air has a price.

A new invention, the CoinAir Machine, is being used by at least six D.C. gas stations. For 25 cents, customers can get four minutes of compressed air delivered to their tires from the coin-operated air hose on the machine.

The CoinAir Machine is a portable unit which can be easily rolled into place of removed for storage. In those stations that are using it, the CoinAir has replaced the traditional free air tower.

Service station operators currently using the new machine say it is not a profit-making scheme. Its purpose, they say, is to curtail the flow of traffic coming into the stations by nonpaying customers. Customers who pay for gasoline or other services get free air, they say.

The operators say they were prompted to use the CoinAir because many of the motorists who come to their stations for air already have bought gasoline at one of the many stations that offer no other services than the sale of gasoline. They also claim that regular air hoses are damaged or stolen and that the free air pumps use costly electricity.

One station owner, who asked that his name not be used, said, "The stations that offer only gas and no other services such as air, paper towels and bathrooms don't save the customers anything - what have you saved when you can't go to the bathroom? It's pathetic."

Leroy Jackson, manager of Fort Dupont Shell Service Station at Massachusetts and Alabama avenues said, "At first I didn't agree withe idea, but then after a while I saw that there was a need for the machine. In the summer before the CoinAir machine was put here, there used to be a line to get to the air pump. Neighborhood kids would be in and out, running up and down the station."

Jackson, who has been in the gas station business for eight years, said some customers had complained about the idea of charging for air. But, he added, "If they are in need, I allow them to use the air free. Customers don't pay for air, it's just done for people who don't buy anything."

One motorist, Bakri Deen of Fort Stevens Drive NW, said he was unhappy with the machine. "I wouldn't want to pay for air when it has been free all the time. But if I needed it, I'd pay for it."

Whether the CoinAir Machine will catch on in the D.C. area is uncertain.

Victor Rasheed, executive director of the Greater Washington-Maryland Service Station Association, said, "I don't see any big rush by dealers to use them. We don't have many stations in the Washington area using the CoinAir Machine, because dealers are conscientious, they don't want to charge the customers for service that has been a traditional responsibility. Dealers are paying for the air themselves. These little extras have been an integral part of the service station."

"It's just a ripoff, like map machines. It takes so little to run air - about 5 cents a day," said Ken Kadala, owner of an Amoco station on Georgia Avenue NW.

The profit from the machine is low. It is split 50-50 with the company which manufactures the machine. An estimated 15 to 25 people use the machine daily in most stations, the operators said.

Said one station owner, who is opposed to the machines: "You've got to give something away free."