The humble plastic water bottle may be a danger to your health

Plastic water bottles have become an indispensable companion of the modern era, helping us stay hydrated while on the go. But are we getting more than just water in our reusable plastic bottles?

For years, rumours have swirled about the health dangers of reusing PET bottles.

Plastic not so fantastic?

A coding system is used on plastic bottles inside the recycling logo to identify the type of plastic it's made from. The water bottles we buy at corner stores or vending machines are made of lightweight plastic called PET or code 1.

"PET is not one of the plastics causing any problems," such bottles are only meant to be used once.

Refillable water bottles are commonly made of stronger polycarbonate, indicated by code 7. Over time, these slowly break down, releasing the chemical Bisphenol A (BPA) into the water, especially if the bottle is repeatedly heated – such as in a dishwasher.

It has been known since the 1930s that BPA mimics the female hormone oestrogen, which now routinely appears in urine samples among the general population – but in amounts 1000 times lower than levels considered dangerous.

BPA is shown to cause cancer, diabetes and reproductive disorders in animal trials, yet no effects in humans have been shown.

Effects have been established using very high levels of exposure on animals. Exposure in humans is probably not sufficient to cause any such health problems.

What to be wary of

Nevertheless, BPA is commonly used in baby bottles and is shown to leach when repeatedly heated or washed. Baby bottles containing BPA have been withdrawn in the USA and elsewhere, and health authorities here are under pressure to follow suit.

People feeding young children are saying they'd rather not take the chance, which is reasonable enough. Avoiding bottles coded 7, and says those with codes 2, 4 and 5 are the best choices for reuse.

These numbers indicate the plastic is either polyethylene or polypropylene. There's absolutely nothing in these that could be classified as dangerous, so be reassured."

The other plastic codes are 3 for polyvinyl – not used with food – and 6 for polystyrene, mainly used in disposable cups and egg cartons.

Number 7 bottles are also the only ones where unhealthy leaching is accelerated by heating. Sunlight has no harmful effect, with scientific studies showing sunlight affects only the bottle exterior, not the interior or contents. However, like heating, sunlight ageing eventually causes small cracks, an indication that your bottle is past its re-use date.

Plastic dos and don'ts

• Don't heat plastic bottles by filling them with hot liquid or putting them in the microwave or dishwasher.
• Use glass bottles where practical. Steel and aluminium are good alternatives.
• Avoid bottles coded 7, especially for children.
• Choose clear plastics, as colouring agents do leach into water.
• Replace bottles when they show signs of cracking.