Why would a beaker of distilled water and NaOH need to be covered?

I needed to drop the pH level of the NaOH (base) with a pH level of 10 to a pH level of 7 by blowing (CO2) into the beaker after i rested for 1 minute but while that 1 minute was going by, we needed to make sure the beaker of NaOH was covered. Why is this?