Acid rain most likely.
You can't change your pH levels.
Buffers are solutions designed to resist changes to pH levels.
Different parts of the skin tend to have different acidity levels which is why the ph levels change.
All enxymes work at an optimum pH. Changes in pH levels could change the rate at which the enzymes work by denaturing them.
Different levels of pH
6 to 8 extream ph levels such 2, 11 causes irritating skin...
pollutions, chemicals
Factors like decomposition and absorption of gas from the atmosphere can effect pH. Because these factors are rarely constant, pH will change over time.
Elevated levels of Hydrogen Ions, which makes it have a low pH.
they keep pH levels constant with minimal change
Neon is a monatomic gas, it dissovles slightly in water but causes no change to the pH as it is completely inert.
You can make a simple pH indicator at home using red cabbage. There are a few other plants that will change colours in different pH levels.