Sputum, which is mucus that is coughed up, contains various substances such as serum proteins, lipids, electrolytes, Ca++, and DNA from degenerated white blood cells. It also includes extra secretions from the nasal, oral, lingual, pharyngeal, and salivary glands. The color of sputum is more closely related to the amount of time it has been present in the lower respiratory tract rather than the presence of infection. While not definitive for diagnosing diseases, the color of sputum can provide information about the diagnosis and stage of the disease. Mucus can have different colors including white, yellow, green, red, pink, brown, gray, and black. The color is often determined by the type of cells present in the airways and how long they have been there. Clear or cloudy white thin mucus is considered normal, but if there is an increase in amount and thickness, it may indicate an early sign of infection. Thick white mucus can be a prominent feature of gastroesophageal reflux disease caused by gastric acid reflux into the airways. Yellow and green coloration of mucus is due to the presence and breakdown of neutrophils and eosinophils in infectious and allergic diseases. Yellow is usually associated with more acute disease, while green indicates a more chronic stage with the presence of bacteria. Red mucus indicates the presence of red blood cells in the airways and is associated with conditions such as pneumococcal pneumonia, lung cancer, tuberculosis, and pulmonary emboli. Pink mucus is typically associated with the breakdown of eosinophils in individuals with allergies. Gray, brown, and black mucus is often linked to cigarette or marijuana smoking, cocaine use, air pollution in workplaces such as coal mines, and old blood.

Cilia, which are hair-like structures, play a crucial role in moving the mucus in the respiratory tract. They beat at a rate of 1000 strokes per minute, with a powerful forward stroke and a slow return or recovery stroke. During the forward stroke, the cilia extend upwards into the sticky mucus layer, moving it along with any trapped particles. On the reverse beat, the cilia release the mucus and retreat completely into the watery sol layer. In the nasopharynx, cilia beat in a direction that propels the mucus into the pharynx, while in the trachea, they propel mucus upwards towards the pharynx, where it is swallowed.

The deposition of particles in the lungs depends on various factors including the size, density, and shape of the particles, the distance they need to travel, airflow speed, and the relative humidity of the air. There are four main mechanisms of particle deposition: impaction, sedimentation, interception, and Brownian movement. Impaction occurs when particles larger than 10 µm are deposited in the nasal passages and do not penetrate into the lower respiratory tract. Sedimentation is the deposition of particles with sizes ranging from 0.2 to 2.0 µm. Interception refers to particles that are intercepted by the airway surfaces. Brownian movement leads to the deposition of particles smaller than 0.2 µm.
Sputumisexpectoratedmucus.However,inadditiontomucus,sputumcontainsserumproteins,lipids,electrolytes,Ca++ ,DNAfromdegeneratedwhitebloodcells(collectivelyknownasbronchial secretions),andextrabronchialsecretions;includingnasal,oral,lingual,pharyngeal,andsalivarysecretions.Thecolorofsputumismorecloselycorrelatedwiththeamountoftimethatithasbeenpresentinthelowerrespiratorytractthanwiththepresenceofinfection.Althoughnotpreciselyidentifiablewithdiseasediagnosis,thecolorofsputumcanbeinformativeinhelpingleadtoadiagnosisandstageofdisease.Mucushasmanycolors:white,yellow,green,red,pink,brown,gray,andblack.Thecolorationiscommonlyduetothetypeofcellpresentintheairways(inflammatorycells,suchasneutrophilsoreosinophils,orredbloodcells)andhowlongtheyhavebeenthere.Clearorcloudywhite thinmucusisconsiderednormal;however,ifamountsandthicknessareincreased,itmayrepresentanearlysignofinfection.Thickwhitemucuscanbetheonlyidentifiablefeatureofgastroesophagealrefluxdiseasecausedbygastricacidrefluxintotheairways.Yellow andgreen colorationofmucusisduetothepresenceandbreakdownofneutrophilsandeosinophilsininfectiousandallergicdiseases.Yellowistypicallyassociatedwithmoreacutedisease(infection,allergy)andgreenusuallyindicatesamorechronicstagewiththepresenceofbacteria(chronicbronchitis,bronchiectasis,cysticfibrosis,andlungabscess).Red mucusindicatesthepresenceofredbloodcellsintheairwaysandisassociatedwithpneumococcalpneumonia,lungcancer,tuberculosis,andpulmonaryemboli.Pink mucusistypicallyassociatedwiththebreakdownofeosinophilsinindividualswithallergies.Gray,brown,andblackmucusisoftenassociatedwithcigaretteormarijuanasmoking,cocaineuse,airpollution(workplaceenvironment,suchascoalmines),andoldblood.
1000 strokes per minute, with a power forward stroke and a slow return or recovery stroke. During their power forward stroke, the tips of the cilia extend upward into the viscous mucus layer and thereby move it and the entrapped particles. On the reverse beat, the cilia release the mucus and withdraw completely into the sol layer. Cilia in the nasopharynx beat in the direction that propels the mucus into the pharynx, whereas cilia in the trachea propel mucus upward toward the pharynx, where it is swallowed.
In general, deposition of particles in the lung depends on the particle’s size, density, and shape; the distance over which it has to travel; airflow speed; and the relative humidity of the air. The four major mechanisms for deposition are impaction, sedimentation, interception, and brownian movement. Particle characteristics and properties, which influence the mechanism of deposition, are listed in 26.2 . In general, particles larger than 10 µm are deposited by impaction in the nasal passages and do not penetrate into the lower respiratory tract. Particles 2 to 10 µm in size
Sedimentation0.2to2.0InterceptionNABrownianmovement<0.2
NA,notapplicable.