Acidity is a measure of how many hydrogen ions are floating around in a liquid. The more hydrogen ions present, the more acidic the solution. Scientists quantify this using the pH scale, which runs from 0 (extremely acidic) to 14 (extremely basic or alkaline), with 7 being perfectly neutral. Every liquid you encounter, from coffee to blood to ocean water, sits somewhere on this scale.
How the pH Scale Works
The pH scale is logarithmic, which means each whole number represents a tenfold change. A solution with a pH of 3 is ten times more acidic than one with a pH of 4, and a hundred times more acidic than one with a pH of 5. This is why small shifts in pH can have outsized effects on living systems.
A solution is acidic when its hydrogen ion concentration exceeds its hydroxide ion concentration, giving it a pH below 7. A solution is basic (or alkaline) when the reverse is true, pushing the pH above 7. Pure water sits right at 7 because its hydrogen and hydroxide ions are present in equal amounts.
Some everyday reference points: lemon juice lands around pH 2, black coffee around 5, milk near 6.5, baking soda dissolved in water around 8.3, and household bleach near 12.5.
Acidity Inside Your Body
Your body maintains different levels of acidity in different places, each tuned to a specific job. Your stomach is one of the most acidic environments in the animal kingdom, with a pH between 1.5 and 2.0, comparable to the stomach acid of scavengers that eat decaying meat. That intense acidity serves two purposes: it breaks down proteins in food (especially animal-derived food, which gastric acid and digestive enzymes can completely decompose) and it kills harmful bacteria before they reach the intestines. Your body invests significant energy producing and containing this acid.
Your blood, by contrast, operates in an extremely narrow pH window of 7.35 to 7.45, making it slightly alkaline. Even small deviations outside this range can disrupt cell function. To keep blood pH stable, your body relies on buffer systems, the most important being the bicarbonate buffer. Bicarbonate acts like a chemical sponge, soaking up excess hydrogen ions when acidity rises and releasing them when it drops. Your lungs and kidneys work together to fine-tune this balance, with the lungs adjusting how much carbon dioxide you exhale and the kidneys filtering excess acid or bicarbonate through urine.
When Blood Acidity Goes Wrong
When blood pH drops below 7.35, the condition is called acidosis. There are two main types, distinguished by their cause. Respiratory acidosis happens when your lungs can’t expel enough carbon dioxide. The excess CO2 dissolves in blood, forming carbonic acid that lowers pH. This can occur with severe lung disease, airway obstruction, or anything that impairs breathing. Metabolic acidosis, on the other hand, results from either an overproduction of acid in the body or the kidneys’ inability to remove enough of it. Causes include uncontrolled diabetes, kidney failure, and severe dehydration.
The body tries to compensate for one type by adjusting the other system. If a lung problem is driving acidity up, the kidneys gradually retain more bicarbonate to offset it. If a metabolic problem is the cause, the lungs speed up breathing to blow off more CO2. When compensation follows the expected pattern, the problem is straightforward. When it doesn’t, both systems may be affected at once.
Acid Reflux and Esophageal Acidity
One of the most common reasons people search about acidity is heartburn and acid reflux. Your stomach’s lining is designed to handle pH levels near 2, but your esophagus is not. When stomach acid flows backward into the esophagus, the tissue becomes irritated and inflamed, producing that familiar burning sensation behind the breastbone.
Doctors diagnose chronic acid reflux (GERD) partly by measuring how long the esophagus stays at a pH below 4 over a 24-hour period. In a healthy person, esophageal pH stays above 4 nearly all day. In someone with pathological reflux, pH frequently dips below 4 with significant fluctuations. The total percentage of time spent below that threshold is one of the most reliable markers separating normal reflux from a condition that needs treatment.
Treatment works on two different fronts. Antacids neutralize acid that’s already been produced, offering quick but short-lived relief. A second class of medications works by blocking the signal that tells stomach cells to produce acid in the first place, reducing the volume of acid at its source. These take longer to kick in but provide a longer duration of relief. Some people use both together for immediate comfort while waiting for the longer-acting medication to take effect.
Food Acidity vs. Metabolic Effect
A common source of confusion is the difference between a food’s pH and its effect on your body’s acidity after digestion. Lemons, for example, are highly acidic in the mouth (pH around 2) but produce alkaline byproducts once metabolized. Meat and cheese, which aren’t particularly acidic on a plate, generate acid when broken down.
Researchers measure this using a metric called the potential renal acid load (PRAL), which estimates how much acid or base your kidneys have to deal with after you digest a particular food. Foods high in protein and phosphorus (meat, dairy, grains) tend to have a positive PRAL, meaning they increase the body’s acid burden. Fruits and vegetables generally have a negative PRAL, meaning they reduce it. This distinction matters more for long-term dietary patterns than for any single meal, since your buffer systems handle short-term fluctuations easily.
Acidity in the Environment
Acidity plays a major role beyond the human body. The ocean absorbs roughly 30% of the carbon dioxide released into the atmosphere. When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid, which then releases hydrogen ions, lowering the ocean’s pH. This process, known as ocean acidification, has a cascading effect on marine life.
The core problem is that excess hydrogen ions bond with carbonate ions, pulling them out of circulation. Carbonate ions are the building blocks that shellfish, corals, and other marine organisms use to construct their shells and skeletons. As fewer carbonate ions remain available, these organisms struggle to grow and maintain their structures. The ocean’s average pH has already dropped measurably since the Industrial Revolution, and the shift is accelerating alongside rising CO2 emissions.