Coping with radon. The discovery that a hazardous natural gas is seeping in unsafe quantities into many homes across America has government agencies and businesses at work seeking ways to determine the problem, and solve it. Home sales likely to require tests

From trickle to stream ... and then a flood? That's the way the number of radon inspections of United States homes has grown in recent years - and is likely to continue to grow. The reasons go beyond health concerns about the long-term effect of breathing radon, an odorless gas caused by the natural decay of uranium found in the earth. They also have to do with home sales, liability of sellers, and the growing concern of banks and secondary mortgage companies that their investments in these structures be protected.

Three years ago there were few radon tests and hardly any radon specialists in America. Now many thousands of tests are taken yearly, inexpensive testing devices are sold in hardware stores, radon testing-and-repair organizations have sprung up, and a number of physicists and geologists have become extremely knowledgeable about the gas. The trend is likely to accelerate now that the Environmental Protection Agency and the US Public Health Service have identified radon as a serious health hazard and have urged that all American homes be tested.

``Right now the majority of radon tests are done as part of a real estate transaction,'' says Richard Mansfield III, consultant to the Employee Relocation Council. (Rising home-inspection costs, P. 12.)

Ruth Dickey, manager of the Bethesda-Chevy Chase (Md.) office of Merrill Lynch Realty, says that in real estate transactions handled by members of her office radon-inspection clauses are ``at the stage where home inspections were 10 to 12 years ago'' - about half of home buyers demand them. Other brokers say their percentages are lower, but most agree the trend is up - and going higher.

Since the EPA report was released late last month, ``we haven't had a contract come across the desk'' without a radon-inspection clause, Ms. Dickey says. ``Within 10 years,'' she adds, radon clauses ``will be in 90 percent of the contracts. [Eventually] they'll probably be standard, like termite inspections.''

Today's usual radon clause, Dickey says, obligates sellers to make repairs before completion of the sale. Radon specialists say that in the heavy majority of cases the cost is between a few hundred dollars and $2,000. ``The most expensive we've had,'' says Dickey, ``is a $1,200 charge to a seller.''

Elevated radon levels, Mansfield says, ``are going to affect value'' of homes. Further, ``in a down market,'' he says, ``they're going to be a negotiating chip'' in setting the price of a house.

That's of concern to banks, too, and to the big secondary-mortgage companies that buy up many mortgages from banks and resell some, after first repackaging them into kinds of securities.

``I would say,'' says George Bauman, ``that in two or three years we'll see'' that banks and secondary institutions will require radon tests before closing. Mr. Bauman, legal counsel to the Montgomery County (Md.) Board of Realtors, says that some are already studying the possibility.

The two most influential secondary-mortgage firms are the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation (familiarly known as Fannie Mae and Freddie Mac). Neither requires radon inspections on single-family dwellings for which it holds or processes mortgages. Further, John Hemschoot, director of Freddie Mac's home mortgage standards, says his organization is ``not considering a policy to require a radon inspection.''

He does says, however, that Freddie Mac is ``working on standards to address environmental risks in general'' - radon, hazardous wastes, and asbestos, among others. These standards, expected to be completed ``by the end of this year,'' relate to evaluations by appraisers of the worth of the property - an evaluation fundamental in determining a loan.

What standards Freddie Mac arrives at are important. Its standards, and those of Fannie Mae, ``are recognized as the standards in the banking industry,'' Mr. Hemschoot says.