Lock box keys, which allow real estate agents and brokers to enter houses when the owners are not at home, are also available to a number of appraisers and exterminators in the Northern Virginia area, an industry spokesman said yesterday.

The spokesman, Arthur W. Kogstad, executive vice president of the Northern Virginia Board of Realtors, said about 70 appraisres and employes connected with exterminating companies have the keys in addition to about 5,800 real estate agents and brokers in Northern Virginia.

Fairfax County police announced Wednesday that they had traced about 70 burglaries of homes up for sale in the Virginia suburbs to a 37-year-old real estate agent who allegedly used his lock box keys to enter the homes and steal personal property worth up to $300,000.

However, industry spokesmen and George C. Walker Jr., chairman of the Virginia Real Estate Commission, which regulates the industry in the state, said in separate telephone interviews yesterday that the lock box system has worked well in the past and that there is no cause for concern and among homeowners planning to sell their houses.

"Our requirements are probably a whole lot higher than they are in other states," Walker said of the training process potential agents or brokers go through before they receive a license. The commission chairman said Virginia employs 12 full-time investigators and is in the process of hiring six others to check into complaints about the industry.

A spokesman for the National Association of Realtors said in a telephone interview from Chicago that the case of Michael Montrose, the Falls Church man accused of the Northern Virginia burglaries, was "a great exception" but that the association had recently urged local boards of realtors to ensure that their systems are secure.

Robert L. Connor, president of the Metro Pest Control Association, said that exterminator operators can get the keys only if the operators are bonded and that the keys are signed out by workers when they go to the houses. "If anything happens to that house, they're responsible," Connor said of the exterminators.

Industry spokesmen said, however, that real estate firms are not liable for stolen goods or damage done to a house during the period that the house is equipped with a lock box.

Before a person can become a real estate agent in Virginia he must take 60 hours of classroom instruction and pass an examination prepared by the Educational Testing Service in Princeton, N.J. Once these requirements are met, the State Real Estate Commission must approve the certificate.