Why is it true to say that exhaled air is acidic?

Because it contains carbonic acid

Explanation:

Carbonic acid is made from dissolving carbon dioxide in water. Since carbon dioxide is exhaled in the lungs, and the exhaled air is moist due to the presence of water molecules from the alveoli, then the carbon dioxide gas will be dissolved in the exhaled air’s moisture rendering it  slightly acidic.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.