A July 22 Associated Press story about the latest efforts to restore the Sphinx in Egypt repeats some popular misconceptions about its deterioration, particularly that some of modern man's activities (pollution of air and underground water) are contributing to it.
Not so. The cause of the Sphinx's decay is soluble salts, as the article, to its credit, briefly mentions. Sodium chloride and calcium sulfate are dissolving and recrystallizing with changes in the area's relative humidity and are exerting cyclic stresses on the limestone. These salts are of geologic origin and affect many monuments in Egypt.Concerning the article's statement that the Sphinx is suffering due to air pollution and "dirty underground water," some have supposed that exhaust from tourist and other vehicles is a threat. Since internal combustion engines do not emit significant amounts of sulfur dioxide (the air pollutant that attacks limestone), this cannot be. Acid rain can also not be involved, partially because there is almost no rain in that part of Egypt, but more because the rain would be alkaline given the nature of the dust.
Dirty underground water is apparently not a problem since the distribution of damage to the Sphinx isn't consistent with the usual pattern of rising groundwater. Also, salts characteristic of wastewater, such as nitrates and phosphates, have not been observed.
It appears this is one problem that "civilized" man is not responsible for.
Steven L. Wilde