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Influenza H1N1 (Swine Flu), Avian Flu, and flu pandemic
Public health officials have been concerned about the risk of another pandemic influenza. Recent concerns about the spread of H1N1 have increased interest in making facemasks available to large numbers of the general population. Face masks can reduce the spread of airborne disease but may not be readily available to everyone in the event of an outbreak or pandemic. The World Health Organization (WHO) has advised that people handling birds during a bird flu epidemic should cover their mouths with a cloth if a facemask is not available. 
Home-made Mask Studies
Studies have evaluated the effectiveness of different homemade masks. While none of these are as effective as a commercially designed face mask, they all offer some protection when other options are not available.
"Home-Made Mask" (Tea cloth)
Evaluation of effectiveness of three masks; a home-made mask made from a tea cloth, a surgical mask, and Filter Facepiece Against Particles (FFP2). Subjects performed a variety of different physical actions. Study included both adults and children. 
- FFP2 provided 50 times as much protection as home-made mask.
- Surgical mask provided 25 times as much protection as home-made mask.
- "Masks worn by patients may not offer as great a degree of protection against aerosol transmission."
- Children were less protected.
- Better at "inward protection"
"Simple Respiratory Mask" (8 layer t-shirt)
A home-made mask using eight layers of 2-ply t-shirt was tested for effectiveness and fit against a N95 mask. 
- N95 masks requires fit factor of 100, this mask had a fit factor of 67
- "offered substantial protection from the challenge aerosol and showed good fit with minimal leakage."
- May be less effective when made by naïve users
- May be uncomfortable
- "No easy, definitive, and affordable test can demonstrate effectiveness before each use.”
1 or 2 layer T-shirt or cravat
Animals exposed to the bioterrorism agents Ricin and Saxitoxin were more likely than control animals to survive when protected with one or two layer t-shirt or cravat. 
Infection Control for Viral Haemorrhagic Fevers In the African Health Care Setting "If surgical masks are not available, use cotton masks made from four or five layers of cotton cloth sewn together. Use a different colour for each side of the mask. This will help health care workers quickly identify which side should be worn inside."
Masks for Influenza Patients: Measurement of Airflow from the Mouth INOUYE SAKAE et. al. Jpn J Infect Dis 59, 3, 179-181 (2006) A study of the effectiveness of three simple commercially available (in Japan) face masks to reduce air speed of coughing and blowing. All masks, including a 16-ply, 10.7 g weight cotton mask reduced air speed of cough to less than 1/10.
- Advice for people living in areas affected by bird flu or avian influenza WHO, 8 November 2004
- Professional and Home-Made Face Masks Reduce Exposure to Respiratory Infections among the General Population Marianne van der Sande1, Peter Teunis1, Rob Sabel PLoS ONE 3(7): e2618.
- Simple respiratory mask. Dato VM, Hostler D, Hahn ME. Emerg Infect Dis, 2006 Jun
- Biological Warfare and Terrorism: Recognition, Protection, and Treatment. Darling RG, See pages 47-48.
I once saw a recommendation for a “home-made” mask made out of Indian saree silk. That is silk which is very thin, extremely tightly woven and easy flowing. You can probably get hold of a few yards at the nearest Indian tailor or cloth shop. The description, if I remember correctly, was to fold it until you have 8 layers, then shape in a form that covers mouth and nose. It is nowhere near the quality of an N95 mask, but it is washable and cheap.