I heard that it was not safe to refill plastic water bottles. Do you know why?

Bottled water, prior to you opening it, is sterile. The instant you open it, bacteria enters the bottle and starts to grow. This is not a problem for a short term, the chances of there being an substantial amount of bacteria is almost zero... but after repeated emptying and refilling, more and more bacteria grow it the small amounts of water left and start to grow on the insides of the bottle.

The University of Calgary did a recent study and illustrated the results.

In the study, researchers sampled drinking water collected from the personal water bottles of 75 students. They found significant levels of coliform bacteria (fecal bacteria, probably from improper handwashing after bathroom use) and heterotrophic bacteria (microbes that feed on other organisms) in 9% to 13% of the samples analyzed. Heterotrophic bacteria counts exceeded the limits set for drinking water in 64% of the bottles. The researchers believe the high counts represented bacterial regrowth in bottles that remained at room temperature for an extended period.

The same danger theoretically exists if you refill commercial bottled water containers. An additional drawback is that they are even more difficult to wash and dry due to a narrow neck. It's true that water is not the most likely environment for bacteria to grow in, but since water bottles come in contact with the mouth, particles of food mixed with saliva could flow back into the bottle where--under the right conditions, such as warm temperatures or even room temperature--bacteria could thrive.

As for health concerns beyond bacterial, the possibility that chemicals from PET water bottles could break down and leach into constantly reused bottles is highly unlikely, say both the FDA and the IBWA.

personally, i refill plastic water bottles all the time.

is it scientifically proven? or is it the company wanting to throw out your bottles to buy more? also, it depends on the quality of the plastic used. if the plastic used is failry decent, it will take a very long time to break down.

its probably been proven that the lesser quality plastics degrade. thats probably been proven an infinite amount of times

Another reason I've heard is that when you use a water bottle germs and bacteria constantly enter via your mouth, the air etc. By constantly refilling it your creating a breeding ground for the germs. I think these are just theories, there isn't enough eveidence to prove they're correct. I've been doing it for years and I'm fine(mostly). Beside I hate constantly buying them, wasting money on ordinary water and creating a heap of waste!!!

Scientifically proven

The reason why is because the plastic is cheap and it expires. When it begins to expire particles of the plastic will get into the water and it can be poisinous

Another reason that hasn't been mentioned is because most people don't wash the bottles when they refill them. bacteria gets into the bottle and can cause you to get sick.

Some plastics degrade, and filling them over and over will cause some of the plastic to enter the water. Supposedly.

Of course it's not proven. They want you to buy more water.

theres one side that say "toxins" leak in...

another argument that you just can't sterilize plastic bottles...

I'm thinking, re-use to what level...a month or two...

and I think science is talking "indefinitely"...

It's not true. See link below.