Mucus is the general secretion produced by mucous membranes throughout the body — airways, gut, eyes and elsewhere. Sputum is specifically mucus coughed up from the lower airways and spat out. All sputum is mucus; most mucus never becomes sputum.
The core difference
- mucus — the slippery secretion of mucous membranes anywhere in the body.
- sputum — material coughed up from the lungs and airways and expelled through the mouth.
Location and route
The distinction is where it came from and how it left. Mucus in the nose that is blown out is nasal mucus, not sputum. Sputum specifically comes from below the vocal cords and arrives by coughing. This matters clinically because a sputum sample is meant to represent the lower airway — a sample that is mostly saliva is rejected by the laboratory.
Phlegm sits between them
Phlegm is the everyday word for mucus in the airways, particularly when there is more of it than usual. It is not a precise clinical term. Roughly: mucus is the substance, phlegm is airway mucus in quantity, sputum is what has been coughed out.
Mucous and mucus
A spelling worth getting right. Mucus with a u is the noun — the substance. Mucous with ous is the adjective — a mucous membrane. The membrane is mucous; what it produces is mucus. The same pattern applies to callus and callous.
What mucus is for
It is not waste. Mucus traps particles and pathogens, keeps surfaces moist, and carries antimicrobial proteins. In the airways it sits on a carpet of cilia that sweep it steadily upward, a mechanism called the mucociliary escalator. Smoking paralyses those cilia, which is a large part of why smokers cough more in the morning — the escalator restarts overnight.
Where the words come from
Sputum is Latin, the past participle of spuere, to spit — the same root as English spit and spew. Mucus is Latin too, related to mucere, to be mouldy.
Phlegm comes from Greek phlegma, one of the four humours of ancient medicine, thought to cause a calm and unemotional temperament — which is why phlegmatic still means unexcitable.
This page explains the words, not your symptoms. Anything persistent is a question for a doctor.
Frequently asked questions
What is the difference between sputum and mucus?
Mucus is the general secretion of mucous membranes anywhere in the body. Sputum is mucus coughed up from the lower airways and spat out.
Is phlegm the same as sputum?
Not exactly. Phlegm is the everyday word for airway mucus in quantity; sputum is specifically what has been coughed out, and is the term used clinically.
Is it mucus or mucous?
Mucus is the noun, the substance. Mucous is the adjective, as in mucous membrane. The membrane is mucous; what it produces is mucus.