All Stories

  1. Frequency and factors leading to prolonged hospital stay and in-hospital mortality in patients admitted with acute exacerbation of chronic obstructive pulmonary disease

    Article • Monaldi Archives for Chest Disease, February 2026, PAGEPress Publications

    Muhammad Irfan

  2. Latent tuberculosis infection in high TB disease burden countries dysregulates cellular and immunological profiles which is further enhanced with uncontrolled hyperglycemia

    Article • Scientific Reports, January 2026, Springer Science + Business Media

    Muhammad Irfan

  3. Second-hand smoke and survival in former smokers with lung cancer: time to act

    Article • Lung Cancer, November 2025, Elsevier

    Muhammad Irfan

  4. Risk Factors and Clinical Outcomes of Invasive Fungal Infections in Patients with Severe COVID-19: A Case–Control Study

    Article • Pathogens, October 2025, MDPI AG

    Muhammad Irfan

  5. Xpert MTB/RIF Cycle Threshold as a Marker of Tuberculosis (TB) Disease Severity: Implications for TB Treatment Stratification

    Article • Clinical Infectious Diseases, September 2025, Oxford University Press (OUP)

    Muhammad Irfan

  6. Echocardiographic Dysfunction in Intermediate-Risk Pulmonary Embolism and Its Association with Short-Term Outcomes in a Pakistani Cohort

    Article • Pakistan Heart Journal, September 2025, Pakistan Cardiac Society

    Muhammad Irfan

  7. Prevalence and factors associated with respiratory symptoms in an urban population: findings from BOLD study in Karachi, Pakistan

    Article • Discover Medicine, July 2025, Springer Science + Business Media

    Andre FS Amaral, Muhammad Irfan

  8. Concordance between FVC and FEV6 for identifying chronic airflow obstruction and spirometric restriction in the Burden of Obstructive Lung Disease (BOLD) study

    Article • BMJ Open Respiratory Research, July 2025, BMJ

    Andre FS Amaral, Muhammad Irfan

  9. Frequency of cardiac arrhythmias, the associated risk factors, and in-hospital mortality in patients admitted with community-acquired pneumonia

    Article • Infectious Diseases Journal of Pakistan, June 2025, Medical Microbiology and Infectious Diseases Society of Pakistan

    Muhammad Irfan

  10. Frequency of hypovitaminosis d in patients with uveitis presenting at a tertiary care hospital in a low-middle-income country

    Article • Journal of the Pakistan Medical Association, April 2025, Pakistan Medical Association

    Muhammad Irfan

  11. INHALED THERAPIES FOR COUGH RESOLUTION IN ACUTE BRONCHITIS: SINGLE CENTER CROSS-SECTIONAL STUDY FROM PAKISTAN

    Article • Khyber Journal Of Medical Sciences, April 2025, Khyber Girls Medical College Peshawar

    Muhammad Irfan

  12. Bronchodilator responsiveness and future chronic airflow obstruction: a multinational longitudinal study

    Article • EClinicalMedicine, March 2025, Elsevier

    Andre FS Amaral, Muhammad Irfan

  13. Mortality in chronic pulmonary aspergillosis: a systematic review and individual patient data meta-analysis

    Article • The Lancet Infectious Diseases, March 2025, Elsevier

    Muhammad Irfan

  14. Chest X-ray scoring to screen for chronic pulmonary aspergillosis in patients with a history of pulmonary TB

    Article • The International Journal of Tuberculosis and Lung Disease, February 2025, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  15. Frequency of viral etiology in community-acquired pneumonia

    Article • Monaldi Archives for Chest Disease, January 2025, PAGEPress Publications

    Muhammad Irfan

  16. Lung function and onset of cardiometabolic diseases in the longitudinal Burden of Obstructive Lung Disease study

    Article • BMJ Open Respiratory Research, January 2025, BMJ

    Andre FS Amaral, Muhammad Irfan

  17. ChatGPT takes the FCPS exam in Internal Medicine

    Article • June 2024, Cold Spring Harbor Laboratory Press

    Muhammad Irfan

  18. Prevalence of chronic cough, its risk factors and population attributable risk in the Burden of Obstructive Lung Disease (BOLD) study: a multinational cross-sectional study

    Article • EClinicalMedicine, February 2024, Elsevier

    Muhammad Irfan, Andre FS Amaral

  19. Assessing the Feasibility of a Multifaceted Intervention Package for Improving Respiratory Health of Textile Workers: Findings From the MultiTex Pilot Study in Karachi, Pakistan

    Article • Asia Pacific Journal of Public Health, January 2024, SAGE Publications

    Muhammad Irfan

  20. Clinical characteristics, outcome, and factors associated with mortality of pulmonary mucormycosis: a retrospective single-center study from Pakistan

    Article • Therapeutic Advances in Infectious Disease, January 2024, SAGE Publications

    Muhammad Irfan

  21. The Second International Collaborative Mycology Conference: evidence-informed fight against fungal diseases in Pakistan

    Article • Therapeutic Advances in Infectious Disease, January 2024, SAGE Publications

    Muhammad Irfan

  22. Correlation of gastro-esophageal reflux disease with asthma control and quality of life: a cross-sectional study from a low-middle income country

    Article • Therapeutic Advances in Respiratory Disease, January 2024, SAGE Publications

    Muhammad Irfan

  23. Fungal empyema thoracis, a rare but an emerging entity: a retrospective case series from Pakistan

    Article • Therapeutic Advances in Infectious Disease, December 2023, SAGE Publications

    Muhammad Irfan

  24. Allergic bronchopulmonary aspergillosis: radiological and microbiological profile of patients presented in an outpatient pulmonary clinic in a developing country

    Article • Monaldi Archives for Chest Disease, December 2023, PAGEPress Publications

    Muhammad Irfan

  25. Aspergillus-specific IgG antibodies for diagnosing chronic pulmonary aspergillosis compared to the reference standard

    Article • Clinical Microbiology and Infection, December 2023, Elsevier

    Muhammad Irfan

  26. Cohort Profile: Burden of Obstructive Lung Disease (BOLD) study

    Article • International Journal of Epidemiology, October 2023, Oxford University Press (OUP)

    Muhammad Irfan, Andre FS Amaral

  27. Prevalence and factors associated with respiratory symptoms in an urban population: findings from BOLD study in Karachi, Pakistan

    Article • September 2023, Research Square

    Muhammad Irfan

  28. Non-cystic fibrosis bronchiectasis: a retrospective review of clinical, radiological, microbiological and lung function profile at a tertiary care center of low-middle income country

    Article • Monaldi Archives for Chest Disease, September 2023, PAGEPress Publications

    Muhammad Irfan

  29. Four-Month High-Dose Rifampicin Regimens for Pulmonary Tuberculosis

    Article • NEJM Evidence, August 2023, New England Journal of Medicine (NEJM/MMS)

    Muhammad Irfan

  30. Chronic airflow obstruction attributable to poverty in the multinational Burden of Obstructive Lung Disease (BOLD) study

    Article • Thorax, July 2023, BMJ

    Muhammad Irfan, Andre FS Amaral

  31. Impact of immune status on the clinical characteristics, treatment outcomes and mortality of pulmonary nocardiosis: a retrospective analysis in a tertiary care hospital from a low to middle-income country

    Article • Monaldi Archives for Chest Disease, June 2023, PAGEPress Publications

    Muhammad Irfan

  32. Bilateral pneumothorax in a patient with angiosarcoma of the scalp: a case report

    Article • Journal of Medical Case Reports, May 2023, Springer Science + Business Media

    Muhammad Irfan

  33. GRADE-ADOLOPMENT of clinical practice guidelines and creation of clinical pathways for the primary care management of chronic respiratory conditions in Pakistan

    Article • BMC Pulmonary Medicine, April 2023, Springer Science + Business Media

    Muhammad Irfan

  34. Clinical characteristics, outcomes, and factors associated with mortality in Nocardia pneumonia: 18 years' real-world data from a tertiary care hospital in Karachi, Pakistan

    Article • Respiratory Investigation, March 2023, Elsevier

    Muhammad Irfan

  35. Byssinosis and lung health among cotton textile workers: baseline findings of the MultiTex trial in Karachi, Pakistan

    Article • Occupational and Environmental Medicine, January 2023, BMJ

    Muhammad Irfan

  36. Post-COVID-19 Pneumocystis pneumonia cases from Pakistan: an observational study

    Article • Access Microbiology, January 2023, Society for General Microbiology

    Muhammad Irfan

  37. Diagnostic accuracy of different cut-off values of adenosine deaminase levels in tuberculous pleural effusion

    Article • Journal of the Pakistan Medical Association, December 2022, Pakistan Medical Association

    Muhammad Irfan

  38. Factors predicting mortality among patients with COVID-19 associated hospital acquired pneumonia: Insights from a tertiary care center

    Article • Monaldi Archives for Chest Disease, December 2022, PAGEPress Publications

    Muhammad Irfan

  39. <xhtml:span xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en">Comparison of diagnostic accuracy of Xpert MTB/RIF and Geno type MTBDRplus line probe assay for diagnosis of tuberculous pleural effusion </xhtml:span>

    Article • December 2022, Society for General Microbiology

    Muhammad Irfan

  40. Tuberculosis and diabetes mellitus: Relating immune impact of co-morbidity with challenges in disease management in high burden countries

    Article • Journal of Clinical Tuberculosis and Other Mycobacterial Diseases, December 2022, Elsevier

    Muhammad Irfan

  41. Post COVID-19 sequelae of the respiratory system. A single center experience reporting the compromise of the airway, alveolar and vascular components

    Article • Monaldi Archives for Chest Disease, October 2022, PAGEPress Publications

    Muhammad Irfan

  42. Clinical features, microbiology and lung function in post-TB bronchiectasis compared to other aetiologies

    Article • The International Journal of Tuberculosis and Lung Disease, October 2022, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  43. Differences in gender perception of smokeless tobacco

    Article • The International Journal of Tuberculosis and Lung Disease, August 2022, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  44. Indoor Air Pollution and Respiratory Health in a Metropolitan City of Pakistan

    Article • Journal of Occupational and Environmental Medicine, June 2022, Wolters Kluwer Health

    Muhammad Irfan

  45. Treatment outcome definitions for chronic pulmonary aspergillosis

    Article • European Respiratory Journal, March 2022, European Respiratory Society (ERS)

    Helmut Salzer, Muhammad Irfan

  46. The effect of ivermectin on non-severe and severe COVID-19 disease and gender-based difference of its effectiveness

    Article • Monaldi Archives for Chest Disease, January 2022, PAGEPress Publications

    Muhammad Irfan

  47. Latent M. tuberculosis infection is associated with increased inflammatory cytokine and decreased suppressor of cytokine signalling (SOCS)‐3 in the diabetic host

    Article • Scandinavian Journal of Immunology, December 2021, Wiley

    Muhammad Irfan

  48. Pesticides Use and Impaired Lung Function Among Male Agricultural Farmers in Rural Sindh, Pakistan

    Article • Asia Pacific Journal of Public Health, December 2021, SAGE Publications

    Muhammad Irfan

  49. Allergic bronchopulmonary aspergillosis misdiagnosed as smear negative pulmonary tuberculosis; a retrospective study from Pakistan

    Article • Annals of Medicine and Surgery, December 2021, Wolters Kluwer Health

    Muhammad Irfan

  50. Inhaled corticosteroids use in patients with bronchiectasis: Data from the EMBARC registry

    Article • September 2021, European Respiratory Society (ERS)

    eva polverino, Muhammad Irfan

  51. Post tuberculosis radiological sequelae in patients treated for pulmonary and pleural tuberculosis at a tertiary center in Pakistan

    Article • Monaldi Archives for Chest Disease, August 2021, PAGEPress Publications

    Muhammad Irfan

  52. Diagnostic dilemma in COVID-19-associated pulmonary aspergillosis

    Article • The Lancet Infectious Diseases, June 2021, Elsevier

    Muhammad Irfan

  53. Chronic airflow obstruction and ambient particulate air pollution

    Article • Thorax, May 2021, BMJ

    Muhammad Irfan, Andre FS Amaral

  54. Respiratory pathogens in patients with acute exacerbation of non-cystic fibrosis bronchiectasis from a developing country

    Article • Monaldi Archives for Chest Disease, April 2021, PAGEPress Publications

    Muhammad Irfan

  55. Post COVID-19 tuberculosis: An emerging threat of pandemic

    Article • Monaldi Archives for Chest Disease, March 2021, PAGEPress Publications

    Muhammad Irfan

  56. Prevalence of and risk factors for respiratory symptoms in an adult urban population

    Article • The International Journal of Tuberculosis and Lung Disease, January 2021, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  57. COVID-19 Associated Interstitial Lung Disease - An Emerging Entity

    Article • December 2020, Research Square

    Muhammad Irfan

  58. “Allergic Bronchopulmonary Aspergillosis: Radiological and Microbiological Profile of Patients Presented in an Outpatient Pulmonary Clinic of a Developing Country”.

    Article • November 2020, Research Square

    Muhammad Irfan

  59. Invasive pulmonary aspergillosis in patients with severe influenza

    Article • The International Journal of Tuberculosis and Lung Disease, November 2020, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  60. Aspergillus fumigatus and Aspergillus flavus-Specific IgG Cut-Offs for the Diagnosis of Chronic Pulmonary Aspergillosis in Pakistan

    Article • Journal of Fungi, October 2020, MDPI AG

    Muhammad Irfan

  61. Alternate diagnosis in clinically diagnosed pulmonary tuberculosis patients treated at tertiary care hospital in a high TB burden country

    Article • September 2020, European Respiratory Society (ERS)

    Muhammad Irfan

  62. Clinical characteristics and outcomes of patients with H1N1 influenza pneumonia admitted at a tertiary care hospital in Karachi, Pakistan

    Article • Pneumonia, July 2020, Springer Science + Business Media

    Muhammad Irfan

  63. Factors predicting in‐hospital mortality among patients admitted with community acquired pneumonia at a tertiary care hospital Karachi, Pakistan

    Article • The Clinical Respiratory Journal, April 2020, Wiley

    Muhammad Irfan

  64. Underlying Conditions and Clinical Spectrum of Chronic Pulmonary Aspergillosis (CPA): An Experience from a Tertiary Care Hospital in Karachi, Pakistan

    Article • Journal of Fungi, March 2020, MDPI AG

    Muhammad Irfan

  65. Etiology, Clinical, Radiological, and Microbiological Profile of Patients with Non-cystic Fibrosis Bronchiectasis at a Tertiary Care Hospital of Pakistan

    Article • Cureus, March 2020, Cureus, Inc.

    Muhammad Irfan

  66. Commemorating World TB Day 2020: “IT’S TIME” — It’s time to End the Global TB Epidemic

    Article • International Journal of Infectious Diseases, March 2020, Elsevier

    Muhammad Irfan

  67. Post tuberculosis treatment infectious complications

    Article • International Journal of Infectious Diseases, March 2020, Elsevier

    Muhammad Irfan

  68. Accuracy of genotype MTBDRplus line probe assay in patients with tuberculous pleural effusion: comparison with clinical and culture based diagnosis

    Article • Infectious Diseases, December 2019, Taylor & Francis

    Muhammad Irfan

  69. International prevalence and risk factors evaluation for drug-resistant Streptococcus pneumoniae pneumonia

    Article • Journal of Infection, October 2019, Elsevier

    Muhammad Irfan

  70. Prevalence and risk factors for Enterobacteriaceae in patients hospitalized with community‐acquired pneumonia

    Article • Respirology, August 2019, Wiley

    Dr Pedro J Marcos, Muhammad Irfan, Dr CATIA CILLONIZ

  71. Visual and treatment outcomes of tubercular uveitis: a prospective case series from a referral hospital in Pakistan

    Article • BMC Research Notes, July 2019, Springer Science + Business Media

    Muhammad Irfan

  72. Utility of the microcolony method for evaluation of multidrug-resistant tuberculosis patients in Karachi, Pakistan

    Article • The International Journal of Tuberculosis and Lung Disease, July 2019, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  73. An international perspective on hospitalized patients with viral community-acquired pneumonia

    Article • European Journal of Internal Medicine, February 2019, Elsevier

    Dr Pedro J Marcos, Dr CATIA CILLONIZ, Muhammad Irfan

  74. Epidemiology of asthma and associated factors in an urban Pakistani population: adult asthma study-Karachi

    Article • BMC Pulmonary Medicine, December 2018, Springer Science + Business Media

    Muhammad Irfan

  75. Microbiological testing of adults hospitalised with community-acquired pneumonia: an international study

    Article • ERJ Open Research, October 2018, European Respiratory Society (ERS)

    Muhammad Irfan

  76. Post tuberculosis sequelae in patients treated for tuberculosis: An observational study at a tertiary care center of a high TB burden country

    Article • September 2018, European Respiratory Society (ERS)

    Muhammad Irfan

  77. Prevalence and Etiology of Community-acquired Pneumonia in Immunocompromised Patients

    Article • Clinical Infectious Diseases, August 2018, Oxford University Press (OUP)

    Dr Pedro J Marcos, Muhammad Irfan

  78. Case Definition of Chronic Pulmonary Aspergillosis in Resource-Constrained Settings

    Article • Emerging Infectious Diseases, August 2018, Centers for Disease Control and Prevention (CDC)

    Muhammad Irfan

  79. Burden and risk factors forPseudomonas aeruginosacommunity-acquired pneumonia: a multinational point prevalence study of hospitalised patients

    Article • European Respiratory Journal, July 2018, European Respiratory Society (ERS)

    Dr Marcos I. Restrepo, Dr CATIA CILLONIZ, Dr Pedro J Marcos, Muhammad Irfan

  80. Disseminated hydatid cyst of liver and lung

    Article • BMJ Case Reports, November 2017, BMJ

    Muhammad Irfan

  81. Case of undiagnosed pneumocystis pneumonia (PCP)

    Article • BMJ Case Reports, October 2017, BMJ

    Muhammad Irfan

  82. Association of hypercapnia on admission with increased length of hospital stay and severity in patients admitted with community-acquired pneumonia: a prospective observational study from Pakistan

    Article • BMJ Open, June 2017, BMJ

    Muhammad Irfan

  83. Clinical characteristics and factors associated with mortality in idiopathic pulmonary fibrosis: An experience from a tertiary care center in Pakistan

    Article • The Clinical Respiratory Journal, May 2017, Wiley

    Muhammad Irfan

  84. Chronic pulmonary mucormycosis: an emerging fungal infection in diabetes mellitus

    Article • Journal of Thoracic Disease, February 2017, AME Publishing Company

    Muhammad Irfan

  85. Diagnostic performance of genoType® MTBDRplus line probe assay in bronchoalveolar lavage for pulmonary tuberculosis diagnosis in sputum scarce and smear-negative patients

    Article • International Journal of Mycobacteriology, January 2017, Medknow

    Muhammad Irfan

  86. Global initiative for meticillin-resistant Staphylococcus aureus pneumonia (GLIMP): an international, observational cohort study

    Article • The Lancet Infectious Diseases, December 2016, Elsevier

    Dr Pedro J Marcos, Muhammad Irfan

  87. Clinical manifestations and outcomes of pulmonary aspergillosis: experience from Pakistan

    Article • BMJ Open Respiratory Research, December 2016, BMJ

    Muhammad Irfan

  88. Endobronchial tuberculosis—a review

    Article • Journal of Thoracic Disease, December 2016, AME Publishing Company

    Muhammad Irfan

  89. Assessment of diabetes among tuberculosis patients presenting at a tertiary care facility in Pakistan

    Article • International Journal of Mycobacteriology, December 2016, Medknow

    Muhammad Irfan

  90. Increased Mycobacterium tuberculosis antigen-induced gene expression of interferon-gamma, tumor necrosis factor alpha and interleukin-6 in patients with diabetes

    Article • International Journal of Mycobacteriology, December 2016, Medknow

    Muhammad Irfan

  91. Post-tuberculosis pulmonary function and noninfectious pulmonary disorders

    Article • International Journal of Mycobacteriology, December 2016, Medknow

    Muhammad Irfan

  92. Behaviour change intervention for smokeless tobacco cessation: its development, feasibility and fidelity testing in Pakistan and in the UK

    Article • BMC Public Health, June 2016, Springer Science + Business Media

    Muhammad Irfan

  93. Reasons for failure to quit: a cross-sectional survey of tobacco use in major cities in Pakistan

    Article • The International Journal of Tuberculosis and Lung Disease, May 2016, International Union Against Tuberculosis and Lung Disease

    Muhammad Irfan

  94. Prevalence of tobacco use among women: a cross sectional survey from a squatter settlement of Karachi, Pakistan

    Article • BMC Research Notes, September 2015, Springer Science + Business Media

    Muhammad Irfan

  95. Pulmonary scedosporiosis mimicking aspergilloma in an immunocompetent host: a case report and review of the literature

    Article • Infection, September 2015, Springer Science + Business Media

    Dr Kauser Jabeen, Muhammad Irfan

  96. Endobronchial tuberculosis presented as multiple endobronchial vesicular lesions

    Article • International Journal of Mycobacteriology, June 2015, Medknow

    Muhammad Irfan

  97. Sarcoidosis presented as retroperitoneal and lung mass

    Article • BMJ Case Reports, March 2015, BMJ

    Muhammad Irfan

  98. Compliance with continuous positive airway pressure (CPAP) therapy for obstructive sleep apnea among privately paying patients- a cross sectional study

    Article • BMC Pulmonary Medicine, November 2014, Springer Science + Business Media

    Muhammad Irfan

  99. Differential Early Secreted Antigen Target (ESAT) 6 kDa–induced IFN-γ and SOCS1 expression distinguishes latent and active tuberculosis

    Article • The Journal of Infection in Developing Countries, January 2014, Journal of Infection in Developing Countries

    Muhammad Irfan

  100. Death certificate completion skills of hospital physicians in a developing country

    Article • BMC Health Services Research, June 2013, Springer Science + Business Media

    Muhammad Irfan

  101. Community-acquired pneumonia

    Article • Current Opinion in Pulmonary Medicine, May 2013, Wolters Kluwer Health

    Muhammad Irfan

  102. Expression of M. tuberculosis-induced suppressor of cytokine signaling (SOCS) 1, SOCS3, FoxP3 and secretion of IL-6 associates with differing clinical severity of tuberculosis

    Article • BMC Infectious Diseases, January 2013, Springer Science + Business Media

    Muhammad Irfan

  103. Factors Associated within 28 Days In-Hospital Mortality of Patients with Acute Respiratory Distress Syndrome

    Article • BioMed Research International, January 2013, Hindawi Publishing Corporation

    Muhammad Irfan

  104. BCG vaccination is associated with decreased severity of tuberculosis in Pakistan

    Article • International Journal of Mycobacteriology, December 2012, Medknow

    Muhammad Irfan

  105. Pulmonary functions in patients with diabetes mellitus

    Article • Lung India, January 2011, Medknow

    Muhammad Irfan

  106. Lung function decrement with arsenic exposure to drinking groundwater along River Indus: a comparative cross-sectional study

    Article • Minerals and the Environment, July 2010, Springer Science + Business Media

    Muhammad Irfan

  107. The Efficacy of Spirometry as a Screening Tool in Detection of Air Flow Obstruction

    Article • The Open Respiratory Medicine Journal, June 2010, Bentham Science Publishers

    Muhammad Irfan

  108. CCL2 Responses to Mycobacterium tuberculosis Are Associated with Disease Severity in Tuberculosis

    Article • PLOS One, December 2009, PLOS

    Muhammad Irfan

  109. Differential Live Mycobacterium tuberculosis-, M. bovis BCG-, Recombinant ESAT6-, and Culture Filtrate Protein 10-Induced Immunity in Tuberculosis

    Article • Clinical and Vaccine Immunology, May 2009, ASM Journals

    Muhammad Irfan

  110. Rapidly progressive cryptogenic organising pneumonia presenting as a lung mass

    Article • BMJ Case Reports, April 2009, BMJ

    Muhammad Irfan

  111. Relationship between Circulating Levels of IFN-γ, IL-10, CXCL9 and CCL2 in Pulmonary and Extrapulmonary Tuberculosis is Dependent on Disease Severity

    Article • Scandinavian Journal of Immunology, March 2009, Wiley

    Muhammad Irfan

  112. Drug susceptibility pattern of Mycobacterium tuberculosis in adult patients with miliary tuberculosis

    Article • Tropical Doctor, July 2007, SAGE Publications

    Muhammad Irfan