Polypropylene bottles make up 83% of the market. Polypropylene is one of the most commonly used plastics. When tested, in Trinity College, Dublin, using the high temperature process for sterilising plastic bottles and preparing milk formula, this caused the bottles to shed millions of microplastics and trillions of even smaller nanoplastics. Plastic kettles and food containers also released millions of microplastics per litre of liquid.
Scientists suggest using water boiled in a non-plastic container and then cooled, to rinse the bottle three times after sterilisation, and making formula ina non-plastic container, before pouring into clean bottle. Glass bottles can also be used.
From The Guardian 20 Oct 2020 https://www.theguardian.com/environment/2020/oct/19/bottle-fed-babies-swallow-millions-microplastics-day-study