Many people refill single-use plastic bottles (Plastic #1, PET) as a cheap way to carry water. That bottle was bought with water in it in the first place - what can go wrong? While a single refill in a freshly drained bottle probably will not cause any problem, there can be some issues when it is done repeatedly.
First, these bottles are difficult to wash and are thus likely to carry the bacteria that have started colonizing it the minute you first unsealed it. In addition, the plastic used in the manufacturing of these bottles is not made for long term use.
To make the plastic flexible, phthalates might be used in the manufacturing of the bottle. Phthalates are endocrine disruptors, a major environmental concern, and which can mimic the actions of hormones in our body. Those chemicals are relatively stable at room temperature (as well as when the plastic bottle is frozen), but they can be released into the bottle when the plastic is warmed.
The Federal Drug Administration (FDA) states that any chemical released from the bottle has been measured at a concentration below any established risk threshold. Until we know more, it's probably best to limit our use of single-use plastic bottles and to avoid using them after they have been microwaved or washed at high temperatures.
Plastic (#7, polycarbonate)
The rigid, reusable plastic bottles often seen clipped to a backpack are labeled as plastic #7, which usually means there are made of polycarbonate. However, other plastics can get that recycling number designation.
Polycarbonates have been under scrutiny lately because of the presence of bisphenol-A (BPA) that can leach into the bottle's content. Numerous studies have linked BPA with reproductive health problems in test animals, and in humans too.
The FDA states that so far they have found the levels of BPA leached from polycarbonate bottles to be too low to be a concern, but they do recommend limiting children's exposure to BPA by not heating up polycarbonate bottles, or by selecting alternate bottle options. Plastics containing BPA are no longer used in the United States for the manufacturing of children's sippy cups, baby bottles, and baby formula packaging.
BPA-free polycarbonate bottles were advertised to capitalize on the public fears of BPA and fill the resulting market gap. A common replacement, bisphenol-S (BPS), was thought to be much less likely to leach out of the plastics, yet it can be found in the urine of most Americans tested for it. Even at very low doses, it has been found to disrupt hormone, neurological, and heart function in test animals. BPA-free does not necessarily mean safe.
Food grade stainless steel is a material that can safely be in contact with drinking water. Steel bottles also have the advantages of being shatter resistant, long-lived, and tolerant of high temperatures. When choosing a steel water bottle, make sure the steel is not found solely on the outside of the bottle, with a plastic liner inside. These cheaper bottles present similar health uncertainties as polycarbonate bottles.
Aluminum water bottles are resistant and lighter than steel bottles. Because aluminum can leach into liquids, a liner has to be applied inside the bottle. In some cases that liner can be a resin that has been shown to contain BPA. SIGG, the dominant aluminum water bottle manufacturer, now uses BPA-free and phthalate free resins to line its bottles, but it declines to reveal the composition of those resins. As with steel, aluminum can be recycled but is energetically very costly to produce.
Glass bottles are easy to find cheaply: a simple store-bought juice or tea bottle can be washed and repurposed for water-carrying duty. Canning jars are just as easy to find. Glass is stable at a wide range of temperatures, and will not leak chemicals into your water. Glass is easily recyclable.
The main drawback of glass is, of course, that it can shatter when dropped. For that reason, glass is not allowed at many beaches, public pools, parks, and campgrounds.
However, some manufacturers produce glass bottles wrapped in a shatter-resistant coating. If the glass inside breaks, the shards remain inside the coating. An additional drawback of glass is its weight - gram-conscious backpackers will prefer lighter options.
At this moment, food-grade stainless steel and glass water bottles are associated with fewer uncertainties. Personally, I find the simplicity and lower economic and environmental costs of glass appealing. Most of the time, however, I find drinking tap water from an old ceramic mug perfectly satisfying.
Cooper et al. 2011. Assessment of Bisphenol A Released from Reusable Plastic, Aluminium and Stainless Steel Water Bottles. Chemosphere, vol. 85.
Natural Resources Defense Council. Plastic Water Bottles.
Scientific American. BPA-Free Plastic Containers May Be Just as Hazardous.