"Each Apollo mission increased the mass of the lunar atmosphere by about 30%, and it took several weeks for the atmosphere to return to its natural state. Highly sensitive measurement or communication instruments may be affected by the additional atmospheric particles." [1]
"Studies have also revealed that the balance is very delicate. At present, the solar wind sweeps the lunar atmosphere into space very efficiently, limiting its density to that of a collisionless gas (an exosphere). If the atmosphere were, however, a factor of just 1000 denser, this process would invert itself and the atmosphere would remain stable for tens of thousands of years. Just the Apollo missions to the Moon increased the atmospheric density by a factor of 10 with rocket exhaust (mostly CHON compounds). Large scale human activity on the Moon could push the total mass over the limit and create an artificial atmosphere, pushing the Exobase off the surface and creating a stable, if highly tenuous atmosphere which could threaten precisely the most important asset that the Moon can offer us: its sterility and almost atmosphereless nature." [2]
[1] http://www.tsgc.utexas.edu/tadp/1995/spects/environment.html
[2] https://web.archive.org/web/20050214155703/http://www.iac.es...