The 20% rule: the body tolerates a 20% excess of most things. This includes baseline glucose, baseline total cholesterol, baseline systole, body fat, etc. etc.
Translation: if normal cholesterol is 180, don’t worry about 216. If fasting blood sugar is 70, don’t worry about 84. If normal systole is 100 throughout life, as it is in the Pima Indians, don’t worry about a systole of 120. If normal body fat is about 15%, don’t worry about 18% body fat.
Obvious exception: a 20% increase in the Fahrenheit temperature of the body. Here we meet a real limitation. Accelerating nearly every chemical reaction in the body creates serious imbalances in the levels of the push-pull chemicals. In addition, the well-nourished, otherwise healthy body has mechanisms to keep its temperature below about 105 F, when it has thrown a fever in response to a serious challenge. A temperature beyond that limit blows out those control mechanisms and creates havoc, where no existing control mechanism in the body can be re-exerted to restore the balance in time to save the body.
What about a 20% increase in H+ concentration in the blood (pH 7.40 decreasing to pH 7.32)? Depends on why – if it is due to an increase in lactic acid production by resting muscle, I would be concerned – early warning of degenerate metabolism and all of the problems associated with it.