This useful article published in 2013 in Disaster Medicine and Health Preparedness explores the effectiveness of home made masks in protecting against an influenza epidemic. They investigated materials ranging from a cotton T shirt to a vacuum cleaner bag.
Their work suggests that although a surgical mask was 3 times more effective than a home made one, if commercially made masks are unavailable, a home made mask was better than nothing, so long as it doesn’t lull the wearer into a sense of complacency. The best masks were made from a pillow case or a cotton T shirt, with the T shirt being slightly prefered because the stretchy fabric made it more comfortable to wear.
These homemade masks could be useful in a last resort if you need to care for someone who’s infected and coughing, but they emphasise that they will provide very little protection against very fine aerosol particles, and should only be used if commercial ones are unavailable.
Remember: all the medical advice says that regular hand washing is much more important than wearing a mask.
Download the full 2013 article here