In addition acidity is present in nature, so that apart from special circumstances (for example, rain is more acidic in volcanic areas), abnormality is only suspected if an area's rain has a long-term acid-base value of less than 5 (each digit less on the scale represents a 10-fold increase in acidity).
加上自然界本身也有一些酸性物質，因此沒有特殊狀況（如火山地區雨水較酸），一地雨水酸鹼值若長期持續小於五（酸鹼值差一，表示酸鹼度相差十倍），才會被懷疑是不正常。