Measuring whether oxygen is low
Before giving oxygen we check whether it is actually low. The simplest tool is pulse oximetry — a clip on the finger that estimates oxygen saturation (SpO₂), the percentage of haemoglobin carrying oxygen. A healthy value is usually 95–100%. A blood test, the arterial blood gas, measures the PaO2 more precisely. A low blood oxygen level is called [[hypoxemia|hypoxemia]], and it is what oxygen therapy is meant to correct — not breathlessness itself, which can occur with normal oxygen levels.
Who benefits, and the danger of too much
Supplemental oxygen genuinely helps when blood oxygen is low. For people with severe COPD and chronic hypoxemia, long-term oxygen therapy used at least 15 hours a day is one of the few treatments shown to lengthen life. But oxygen is a drug with a target range, not a comfort blanket. Giving high-flow oxygen to someone who is not hypoxemic does not help and can harm.
In some people with advanced COPD, the breathing drive has adapted to chronically high carbon dioxide. Flooding them with too much oxygen can blunt that drive and worsen carbon dioxide retention, making them drowsy and acidotic. So in at-risk patients, clinicians aim for a lower target band (often around 88–92%) rather than the 94–98% used for most others. The lesson is the same: titrate oxygen to a target, do not simply maximise it.
Reading a pulse oximeter (SpO2) — a rough guide 95-100% normal in a healthy adult 94-98% usual target band for most patients on oxygen 88-92% target band for CO2-retainers (e.g. some COPD) < 88-90% hypoxemia — needs assessment / oxygen < 90% persistently at rest -> consider long-term O2 Key idea: oxygen is dosed to a TARGET RANGE. Too little -> hypoxemia (organs starved of O2) Too much -> wasteful; in some, worsens CO2 retention Note: SpO2 is an estimate. Cold hands, nail polish, low blood flow, and certain conditions can fool it.
How oxygen is delivered
At home, an oxygen concentrator is the workhorse: it plugs into the wall and pulls oxygen out of room air, so there is nothing to refill. For going out, small cylinders or portable concentrators allow mobility. Oxygen usually flows through soft nasal prongs (a nasal cannula) for low flows, or a face mask for higher flows. In hospital, high-flow nasal oxygen can deliver large, warmed, humidified flows that are far more comfortable and effective than a plain cannula for serious hypoxemia.