They aren't very effective if you buy them from a pharmacy. Their holes stop large spit particles from coming out or in, but allow small particles to come in. They don't prevent against swine flu. If you want a good one, get it from a Hospital.
Source(s):
http://www.slate.com/id/2217045/

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Surgical masks are not that good at preventing diseases. They superior for preventing the diseases that you already have if you cough or sneeze. Regardless, they are more of a buffer for people's fears than anything, and offer better protection than nothing.

http://www.slate.com/id/2217045/

Surgical grade respirators (N95) are slightly better but more expensive.

http://www.latimes.com/news/nationworld/nation/la-sciw-swine-masks28-2009apr28,0,4722242.story

They may help more in crowded areas, as long as they are fitted around the face correctly. I can personally attest that they are uncomfortable for anything except short periods of time.
Source(s):
www.cdc.gov/flu/professionals/infectioncontrol/maskguidance.htm
www.cdc.gov/Features/MasksRespirators/
www.cdc.gov/swineflu/masks.htm

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The biggest problem with respirators/surgical masks is that many people don't wear a mask that fits. They aren't very effective if they don't get a proper "seal".

Another thing to consider is that facial hair can drastically reduce the effectiveness of any mask or respirator. In many industrial facilities, facial hair is banned.
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The short answer - any n95 class mask will work fine.
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Surgical masks aren't a completely sealed and filtered system. So they cannot be considered a 100% safe protection. But they can surely lower the amount of viral particles coming into your throat, and being viral charge (quantity of viral particles) a very important factor in getting or not getting sick, it can actually make a difference.

But there is one specific situation where surgical masks are actually very useful in preventing airborne virus diseases. This is on airplanes, where a very low humidity of air makes respiratory walls (mucosae) very dry.

Respiratory walls secrete a thin layer of a mixture of water, lipids and glicoproteins (mucus) where small particles (like viruses) that are inhaled remain attached. Little hair-like structures on the respiratory mucosa (microcilia) then push the mucus towards the mouth, the esophagus and in the stomach, where viruses are inactivated by low pH and enzymes.

Low humidity on airplanes make mucosae very dry, making this defense much less efficient.

Breathing through a surgical mask (or a thick scarf) keeps part of the big amount of humidity in exhaled air coming from the lungs trapped in the mask, which acts like a sort of "humidity buffer", keeping the mucus-microcilia protective system working.
Source(s):
How do I know this? I'm a physician, we are supposed to study this stuff at university.

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If it's not an airborne disease, and you aren't worried about people coming up to you and sneezing into your mouth, it doesn't really do much besides prevent absentminded mouth-touching.
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Thin surgical masks only last approximately one hour as a filter. They then become so drenched in Pathogens, the Pathogens are able to enter the material and you will then be breathing in the pathogen leaving you exposed. There will also be Pathogens so small they will be able to pass through the material of the surgical mask. The only way to avoid inhaling the Pathogen via face masks is to have a properly fitted breathing apparatus called respirators.
Source(s):
Medical, Nursing background

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One thing not mentioned yet is that fact that if you're wearing a mask, you're less likely to put your filthy hands in your mouth/up your nose etc. It makes you more aware of your face and less likely to infect yourself regardless of how many pathogens get filtered.

Also, there is a study out there that says masks (this secondary source doesn't say which kind) does prevent spread of viruses...

http://www.virtualmedicalcentre.com/news.asp?artid=13247
Source(s):
http://www.mayoclinic.com/health/hand-washing/HQ00407

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