Fairfax County's rental housing inventory dropped last year for the sixth straight year, and average monthly rental prices in the county jumped by the largest margin in a decade, according to a new county report.

The figures underline a trend that Fairfax officials and business persons call worrisome: the disappearance of low-priced housing in the county stemming from condominium conversion and dwindling federal funds for publicly assisted housing.

Since 1979, the number of rental units in Fairfax has dropped nearly 13 percent, to 38,950 from 44,555. At the same time, average monthly rents in the county have jumped to $517 from $310. During 1984, rental prices in the county increased an average of $51 a month, the report said. Only 2.2 percent of rental units in the county were vacant last year, a figure industry analysts say is extremely low.

In addition, the report found: "Renter[s] are younger, less likely to be married, have completed fewer years of education and have lower incomes" than owners.

In a number of recent seminars and speeches, local politicians have warned that the shortage of low-priced housing in Fairfax will hamper industrial expansion there by forcing many blue-collar workers and county employes, including police officers, firefighters and teachers, to live outside the jurisdiction.

County officials say they fear that businesses seeking new offices and plants will eventually follow skilled workers to the outer suburban counties where affordable housing is more plentiful.

The County Board of Supervisors is forming a panel of public officials and corporate executives to examine ways of increasing the number of low- and moderate-priced residences. The cheapest rental prices in Fairfax, according to the report, are in the Lee and Mount Vernon districts south of Alexandria, where some of the oldest neighborhoods in the county are located. The average monthly rent in each of those districts is $472.

The most costly area is the Springfield District in the western portion of the county, where the average monthly rent is $587. Rental prices in the county rose faster than those in the Washington metropolitan area, the report said. They rose 7.6 percent in the region compared to 10.9 percent in the county.