Consumer prices in the Los Angeles area rose 0.5 percent in September, but the annual inflation rate held steady at 3.9 percent, according to data released Oct. 11 from the U.S. Bureau of Labor Statistics.

The bureau reported that prices in Los Angeles and Orange counties rose in September to an index level of 268. That’s up 0.5 percent from the August level and 3.9 percent from September of last year.

The index is calculated by comparing current prices for various consumer products to a three-year average from 1982-1984, which the agency assigned as the base period with a value of 100.

The biggest price rise between August and September was apparel, which increased 3.4 percent. The biggest drop of 14 percent was in home natural gas prices. Motor fuel prices rose 1.7 percent over the month.

Motor fuel prices, though, posted the largest increase over the 12-month period ending in September, up 17 percent. That drove the overall energy price index up 12 percent.

The bureau started releasing a separate consumer price index for Los Angeles and Orange County in April. Previously, the agency included Riverside and San Bernardino counties in the Los Angeles region for the inflation data; those two counties now report through their own separate database.

Economy, education, energy and transportation reporter Howard Fine can be reached at [email protected]. Follow him on Twitter @howardafine.