A high level of hypocapnia is readily recognized as it causes dizziness and tingling of the fingers. These extreme symptoms are caused by the increase of blood pH (alkalosis) following the reduction of CO2, which is required to maintain the acidity of the blood. The absence of any symptoms of hypocapnia is not an indication that the diver's carbon dioxide level is within safe limits and cannot be taken as an indication that it is therefore safe to dive. Conservative breath-hold divers who hyperventilate but stop doing so before the onset of these symptoms are likely to be hypocapnic already without knowing it.