Ha...the Susan Syndrome. Technically it isn't safe...while probably not life threatening, sitting there and licking a large quantity of envelope flaps can have impacts on health. For example, as illustrated on Seinfeld(to a fictitious degree), some envelopes use a very cheap glue that can be toxic in large amounts. If you're going to lick a large quantity of envelopes, take a break and rinse your mouth with alot of water periodically. It will help to keep your mouth clean. You may be allergic to the chemicals in the glue as well, which can cause a break out of hives, a swelling of the tongue, or more extreme reactions. This is very rare and probably won't happen, but it is possible. Extensive licking can cause your tongue to dry up, and be more susceptible to scraping and cutting. Try to avoid licking a large amount in one sitting, and stop to get a drink if you feel your tongue dry up. Paper cuts on the tongue are possible, and they HURT LIKE HELL! Don't lick the envelopes if you have alot to do in one sitting. Instead, get a damp, but not soaked sponge. Dab the envelope lightly with the sponge and wet it as needed. You'll save yourself the trouble of all the licking and nasty flavor of the chemical glue, and minimize the already somewhat minimal health risks.

For my two cents, get one of those sponge daubers or one of those cermaic rollers that roll through a bit of water; and don't use your tongue at all. It's much more sanitary, and you avoid the risks that the other answerers have cautioned you about...

I agree with mp116's answer about the possibilities. But, you can also buy these little bottles with a screw on cap that has a little sponge on the end of it, specifically used for sealing envelopes at Staples, Office Depot, or just about any place with office supplies to avoid taking the chance of getting sick!

I don't know about the safety issue but I have an alternative way to seal the envelopes. Any glue stick can be used to rub the glue on the flap of the envelope. Just as quick as those envelope sealers that use water, and you don't have to wait until the water evaporates for the seal to be complete.

It could be dangerous, very toxic if you are unsure of the adhesive ingredients used in the manufacturing.

As an alternative to licking, I purchased a Bingo Dabber at the dollar store, poured out the dyed water and rinse the sponge applicator. Then i filled it with water. It works great! I run it down the envelops glue line and let sit for a few seconds so that the gule can soften and get very sticky, then seal it up. Works great, no mess and no licking toxic glues!