Yes, the Mask is a Must Have tool in your emergency kit. You can select the normal surgical mask or more protective N95 mask.
The mask is very useful during the event of influenza, flu, haze etc. However, to keep better hygiene, one will need to wash the hand frequently with soap or you can used the alcohol type moist tissue. It is also good to keep a small liquid hand sanitizer in the event of flu epidemic.
During this haze event in South East Asia, it is important to wear a mask if you need to conduct any outdoor activities.