WASHINGTON (AP) -- The nation's largest tobacco company knew as early as the 1970s that smokers of light cigarettes took larger puffs that delivered greater amounts of tar, according to a newly released memo.

The 1975 Philip Morris USA correspondence was released by the Senate Commerce Committee in advance of a hearing Tuesday examining the rating system that allows tobacco companies to market cigarettes as regular, light or ultra-light.

The current rating system gives smokers a false sense that cigarettes with less tar and nicotine are healthier, according to a memorandum produced by Democratic congressional staffers.