Over time a wide variety of materials have been used for infant feeding vessels (see History). The materials now most commonly used in baby bottle containers are glass and some types of plastics. Food-grade stainless steel and silicone rubber are also used. Each of these four materials-plastic, glass, silicone and stainless steel-has advantages and disadvantages.[6] The standard materials used in teats/nipples are latex rubber and silicone.[15]
A number of countries have regulations about allowable food contact materials. Ideally, the material making up the bottle should react as little as possible with the material in the bottle. No material is completely inert, but glass and stainless steel are relatively neutral materials which tend to remain stable and not interact with foods.[2] The disadvantages of glass are that it tends to be heavy and can break more easily.[16]
Plastics are lightweight and resistant to breaking.[16] Manufacturers find them easy to form into a variety of shapes.[17] A wide variety of plastics have been developed, some of which are not well understood in terms of reactivity. Some plastics have been found to be reactive with fluids such as breast milk and infant formula. Chemicals such as Bisphenol A (BPA) may "leach" from a bottle into the substance it holds. In addition, plastics may be more likely to break down when heated or cooled, for example, when being heated in a microwave or being boiled to sterilize them.[18][19]
Polycarbonate plastic was frequently used in baby bottles before 2011, and is still used in some countries.[20] Polycarbonates contain Bisphenol A.[21] Since 2008,[2] at least 40 countries have banned the use of plastics containing Bisphenol A in baby bottles due to safety concerns (see Regulation).[22] Bottles made of polycarbonate may be marked as "#7 PC".[23]
Bisphenol S (BPS) and Bisphenol F (BPF) have been used as substitutes for BPA. They are structurally similar. Comparisons of BPA, BPS and BPF have found that these chemicals have similar potency and action to BPA and may pose similar dangers in terms of endocrine-disrupting effects.[24] This has led to criticisms of the chemical industry[25] and for calls to deal with bisphenols in groups, not individually. In 2021, the Canadian government agencies Environment and Climate Change Canada (ECCC) and Health Canada (HC) held consultations with the goal of grouping 343 known BPA analogs and functional alternatives.[26]
Polyethersulfone plastic (PES) does not contain BPA but does include Bisphenol S (BPS).[27] An assessment of a variety of different baby bottles in use in 2016, reported 4 bottles to be of "high concern", 14 bottles to be of "concern"; and only 6 bottles to be of "no concern" These of "no concern" included two polyamide (PA) and two polyethersulfone (PES) bottles, a stainless steel bottle, and one of the 17 polypropylene (PP) bottles tested.[28]
Phthalates, found in polyvinyl chloride (PVC), are another area of concern.[29] Referred to as "everywhere chemicals" because they are so common, phthalates make plastic more flexible, and have been used in pacifiers and nipples or teats for bottles.[30][31] Phthalates have been banned from use in feeding bottles in the EU.[32] In the USA, there have been repeated calls for the removal of phthalates by the U.S. Consumer Product Safety Commission (CPSC) and others.[33][34][29] Their use in children's toys and products was somewhat restricted by the Consumer Product Safety Improvement Act of 2008.[35][36] Plastics labeled #3 may leach phthalates.[36] Latex rubber nipples may contain phthalates, so silicone nipples may be recommended instead. Packaging may indicate whether a product is "BPA-free" or "phthalate-free".[30]
Plastics may degrade over time in other ways, There are concerns that small beads of plastic may be released into fluids from some types of plastic bottles.[37] In 2020 researchers reported that infant feeding bottles made out of polypropylene caused microplastics exposure to infants ranging from 14,600 to 4,550,000 particles per capita per day in 48 regions with contemporary preparation procedures. Microplastics release is higher with warmer liquids and similar with other polypropylene products such as lunchboxes.[38][39][40] In 2022, the first study to examine the presence of plastic polymers in human blood found plastics of multiple types in the blood samples of 17 out of 22 healthy adults tested (nearly 80%).[41][42] Medical experts have suggested reducing exposure to microplastics by not shaking plastic bottles or exposing them to high temperatures. Some recommend using alternative materials such as glass, silicone, or stainless steel.[43]
Baby bottle nipples (teats) made of clear silicone
Baby bottle nipples (also called teats) are typically made from either silicone or latex rubber. When used for nipples, silicone is clear, durable, and slightly harder than latex. Natural rubber latex teats are elastic, tear resistant, and may feel softer. Latex can absorb odors, while silicone does not. Latex can break down if exposed to sunlight. Some people have allergies to latex.[15]