Phlegm

Introduction

Our lungs are continuously exposed to germs (pathogens), particles, and toxic chemicals that are present in the air that we inhale. Yet, they are remarkably resistant to environmental injury. One of the main contributors to this effective defence mechanism is airway mucus. Airway mucus is a gel that is composed of water and mucins.¹ Mucins are heavily glycosylated proteins, in other words, proteins that have several sugar molecules attached to it.¹ The mucus in our airways traps inhaled toxins and pathogens and these are then transported out of the lungs through the combined action of ciliary movement and coughing.¹ Ciliary movement is brought about by the beating of specialised organelles called cilia (tiny, hair-like structures present on the cells that line the airways) that beat in a wave-like pattern to propel pathogens and inhaled particles trapped in the mucus layer out of the airways.³ Paradoxically, while an insufficient mucus barrier leaves the lungs vulnerable to harm, excessive mucus production or impaired clearance plays a central role in the development of many common airway diseases.^1,3

Phlegm (also known as sputum) is produced in the lungs and throat and is slightly thicker than the mucus that is produced in the nose and sinuses.^4,5 In case of disease, the mucus becomes hyperconcentrated or dehydrated, which may lead to thickening.^6,7 Under normal, healthy circumstances, ciliary clearance of mucus is usually adequate to maintain a sterile airway (in other words, an uninfected airway); however, when an infection is present, the cilia fail to effectively clear the mucus and coughing becomes an important back-up mechanism to help the body get rid of excess mucus.^6,7 When a cough is accompanied by the production of phlegm, it is called a productive cough.⁶ Phlegm can make you cough up sputum that may range in colour from clear to brown or black.^4,5

What causes phlegm?

Common colds and the flu are two conditions that can lead to an increased production of phlegm.^4,5 This is a normal symptom.⁴ Coughing up phlegm can, however, result from various causes, ranging widely in seriousness, and these may include:^4,6,7,8

• Infections
• Allergies
• Asthma
• Acid reflux, also known as gastroesophageal reflux disease (or GORD, for short)
• Lung diseases, such as chronic obstructive pulmonary disease (COPD, such as emphysema or chronic bronchitis).

Asthma and COPD are common respiratory ailments that can lead to the overproduction of phlegm. In 2019, asthma affected an estimated 262 million people and caused nearly half a million (455 000) deaths across the globe. COPD is the third leading cause of death worldwide with tobacco smoking accounting for more than 70 % of COPD cases in high-income countries. In low- and middle-income countries, tobacco smoking accounts for 30 to 40 % of COPD cases. In these countries, household air pollution is a major risk factor.⁷

Certain environmental and lifestyle factors can also contribute to the production of phlegm and these may include:⁴

• Smoking.
  • Drinking beverages that contain caffeine or alcohol
  • Dehydration
  • Spending time in a dry environment.

What the colour of phlegm tells us

The colour and consistency of the mucus often provide good clues as to underlying cause and seriousness of the condition.⁴ However, it should not be seen as a hard and fast rule, and it may sometimes still be challenging to determine what is wrong by simply looking at the colour of phlegm.⁵

Clear phlegm

The body often produces clear phlegm to get rid of pollen, pet dander, or other allergens.⁴ A small amount of clear phlegm now and then is considered normal.⁸ However, producing a lot of clear phlegm might indicate that there is inflammation in the airways,⁸ and sometimes, this may be the result of a more serious underlying condition, such as:⁴

• Viral bronchitis
• Viral pneumonia.

Yellow or green phlegm

Yellow or green phlegm may indicate that the body is fighting an infection.^4,8 During the early stages of an infection, phlegm is usually yellow. It then turns green the longer the infection lingers. Causes of yellow or green phlegm include:⁴

• Sinusitis
• Bronchitis
• Pneumonia
• Cystic fibrosis – a genetic disorder that causes mucus in the lungs to become thick and glue-like.⁵

White phlegm

Several conditions can give rise to the production of white phlegm and these include:⁴

• GORD
• COPD
• Viral bronchitis
• Congestive heart failure. If you produce white phlegm accompanied by shortness of breath, fatigue, or leg weakness, this may be indicative of heart failure and you should immediately contact your doctor.

Black phlegm

Melanoptysis is the medical term of coughing up black phlegm. Common causes of melanoptysis include:^4,8

• Smoking
• Inhaling coal dust or other dark materials, particles or toxins
• A fungal infection, known as exophalia dermatididis.

Brown phlegm or phlegm with brown spots

Old blood can make phlegm brown or rust-coloured or can lead to brown spots in phlegm, and this may be the result of:^4,8

• Bacterial bronchitis
• Bacterial pneumonia
• Cystic fibrosis
• Lung abscess
• Inhaling dust or other brown materials, particles, or toxins.

Bloody phlegm, red or pink phlegm

Blood in phlegm is usually indicative of a serious health issue but may also be due to coughing fits (coughing a lot) or because of an infection.^4,8 Common causes include:⁴

• Pneumonia
• Pulmonary embolism – it is when a blood clot that formed elsewhere in the body travels via the blood stream to the lungs and then gets lodged inside the lung
• Congestive heart failure (usually with the production of pink, frothy phlegm, chest pain, sweating, and shortness of breath)
• Tuberculosis
• Lung cancer.

Coughing up blood without phlegm is a medical emergency that requires immediate medical attention. Call an ambulance or go straight to the emergency department of the nearest hospital.⁴

Diagnosis

When the underlying cause is not easily identifiable, sputum analysis can help to establish the diagnosis. Phlegm can be examined grossly as well as under the microscope to aid the diagnosis.⁹

It is often easy and straightforward to collect a sputum sample. Patients are asked to cough up the phlegm and to collect it in a special container. A deep cough sample, often collected early in the morning before eating or drinking, helps reduce potential errors when the final analysis is performed. To obtain such a sample, a person should rinse his/her mouth with clear water for 10 to 15 seconds to eliminate possible contaminants in the mouth. After expelling saliva, the person should take 3 deep breaths and then cough at 2-minute intervals until bringing up some phlegm. The phlegm is then collected in a sterile, sealed container. If one struggles to produce phlegm in this manner, a physiotherapist can perform certain manoeuvres to make it easer.⁹

Clinical diagnostic sputum tests are then performed to identify the causes of lung infections and other diseases. Sputum culture (where any potential germs present in the sputum are grown in the laboratory) is the most commonly performed test when a patient is diagnosed with pneumonia, as it helps identify the bacteria or fungi causing the airway or lung infection.⁹

Treatment

The underlying cause will dictate treatment.^4,6 People with allergies and excessive phlegm production may obtain relief from decongestants, antihistamines or even corticosteroids. Bacterial infections may require antibiotics, while acid reflux may have to be addressed with antacids or proton pump inhibitors.^4,8 It is important to remember that not all infections require antibiotics and that antibiotics only work for bacterial infections, such as pneumonia. They will not help with viral infections, such as a cold or flu.⁸

Sometimes a cough with phlegm will resolve on its own within a few days. However, if it persists for two weeks or more, one should see a doctor.⁴

Useful tips

Coughing up phlegm is an important mechanism whereby the body rids itself of allergens, irritants, and micro-organisms (germs).^4,5 It may become easier to get rid of mucus or to cough up phlegm with the help of the following:^4,5,8,10

• Use a humidifier or vaporiser to ensure that the air in your home as well as in your nose and throat is moist so that it may help to loosen phlegm
• Inhale steam by sitting in a bathroom with a hot shower running for a while, as this can help loosen phlegm
• Apply a warm, moist washcloth to your face
• Stay well hydrated by drinking plenty of water to thin the mucus out (and to prevent dehydration)
• Try a nasal saltwater spray or rinse to help clear the mucus out as this may make breathing easier
• Gargle with saltwater or saline to loosen phlegm
• Use over-the-counter expectorants or mucolytics – medicines to thin the mucus to make it easier to cough it up, such as acetylcysteine (see below)
• Use herbal remedies, such as eucalyptus oil
• Stop smoking, if applicable.

Acetylcysteine is a mucolytic that reduces the viscosity of airway secretions.¹⁰ By reducing the viscosity, the mucus becomes less sticky, and it is easier to cough it up.

When to see a doctor

Make an appointment with your doctor or at a healthcare clinic if:⁴

• Coughing lasts for longer than 2 weeks
• The phlegm is not clear in colour
• You have a fever
• You are wheezing or have difficulty breathing.

References

1. Fahy JV, Dickey BF; Airway mucus function and dysfunction; New England Journal of Medicine; 2010 Dec 2; 363(23): 2233-2247
2. He M, Zhou X, Wang X; Glycosylation: mechanisms, biological functions and clinical implications; Signal Transduction and Targeted Therapy; 2024; 9:194
3. Bustamante-Marin XM, Ostrowski LE; Cilia and mucociliary clearance; Cold Spring Harbor Perspectives in Biology; 2017; 9: a028241
4. Cleveland Clinic; Coughing up phlegm; My.clevelandclinic.org [Internet]; 20 Jan 2023; available from https://my.clevelandclinic.org/health/symptoms/24636-coughing-up-phleg
5. National Institute of Health; Marvels of mucus and phlegm; NIH News in Health [Internet]; Aug 2020; available from https://newsinhealth.nih.gov/2020/08/marvels-mucus-phlegm
6. Hill DB, Button B, Rubinstein M, Boucher RC; Physiology and pathophysiology of human airway mucus; Physiological reviews [© American Physiological Society]; 10 Jan 2022; 102: 1757–1836
7. Abrami M, Biasin A, Tescione F, et al; Mucus structure, viscoelastic properties, and composition in chronic respiratory diseases; International Journal of Molecular Sciences; 2024; 25: 1933 [https://doi.org/10.3390/ijms25031933]
8. Asthma + Lung UK; Phlegm and mucus; asthmandlung.org [Internet]; 31 Mar 2024; available from https://www.asthmaandlung.org.uk/symptoms-tests-treatments/symptoms/phlegm
9. Shen F, Zubair M, Sergi C; Sputum analysis; StatPearls [Internet], NCBI Bookshelf from the National Library of Medicine, National Institutes of Health; 04 May 2025; available from https://www.ncbi.nlm.nih.gov/books/NBK563195/
10. SAHPRA-approved Professional Information for Mucatak 200; dated 20 Apr 2017