Dangers of Heating Foods in Plastics

The following paper below was written primarily for a chemistry class. However, it came to my attention when choosing this topic that I myself did not know WHY plastics were dangerous if consumed. I sought to find out the chemical and physiological reasons.

Interestingly enough, I’ve had many friends who have asked to read this paper. Is perhaps everyone guilty of (or have in the past) microwaved foods with plastic covers/containers/wraps? I was hesitant about posting this paper, however I hope if anything it will help get across valuable information – and so i’ve tried to make it as reader friendly as possible. Hopefully the gist of the following will be understood without necessitating a science/biology/physiology or chemistry background.

The majority of the information has been researched and is not from my personal views but from expirimentation done over the years. I’ve sourced my references at the end for further reading if you are interested.

Please feel free to give me your feedback.

What are plastics?

In general, plastics are a blend of organic and inorganic compounds, of which contain organic polymers (large molecules connected by covalent chemical bonds) based on carbon atom chains that may be combined with oxygen, sulfur, and/or nitrogen (Michigan State University Department of Chemistry, 2012).

Plastic Additives

Organic polymer plastics tend to be blended with additives (fillers, reinforcing agents and plasticizers) for variety of reasons. Fillers are inexpensive materials that make the product lighter and cheaper, while reinforcing agents improve performance and reduce production costs. Plasticizers (phthalates and adipates) allow rigid organic polymers to become more flexible.

Resin Identification Code

Visible on plastic materials are SPI Resin Identification Codes, a set of symbols used to identify the polymer types of different plastics. Developed in 1988 by the Society of the Plastics Industry (SPI), these codes are used internationally for recycling separation purposes (The Plastics Industry Trade Association, 2009). See Appendix A: Table of SPI Resin Identification Codes for more information.

Xenoestrogen in plastics

Xenohormones are naturally occurring or artificially produced compounds that have hormone-like properties also known as endocrine disruptors. These chemicals interfere with the synthesis, secretion, transport, binding, action or elimination of natural hormones in the body (Crisp et al. 1998). At low concentrations (picomolar to nanomolar), adverse health effects were shown in fetal mammals (Yang et al. 2011). The effects do not necessarily appear immediately but are long-term and accumulate over time (Environmental Health Association of Nova Scotia, 2007). In 2009, the Endocrine Society issued its first statement on endocrine disruption stating that there is strong evidence for adverse reproductive outcomes (infertility, cancers, malformations) from exposure to endocrine disruptors along with mounting evidence for effects on other endocrine systems, obesity, metabolism, and insulin and glucose homeostasis (The Endocrine Society, 2009).

Various compounds used to make plastics are harmful because they are often xenoestrogen sources. The most common xenoestrogen sources in plastics include BPAs, PCBs, Phthalates, and Cadmium (D. Kennedy, CLS 306 Men’s and Women’s Health lecture, July 12, 2012).

The following table lists common xenoestrogen sources present in plastics:

Xenoestrogen Compounds in Plastics

Bisphenol A (BPA) -       Used to make polycarbonate polymers and epoxy resins-       Xenoestrogen-       Endocrine disruptor
Polychlorinated Bisphenol (PCB) -       Synthetic chlorinated organic compounds-       Plasticizers contain PBCs-       Toxic effects: high levels of PCBs can have detrimental effects on the liver, endocrine system, skin, immune system, nervous system, reproductive system and fetal and infant development
Phthalates (DEHP etc.) -       Plastic additive (plasticizer) users to make plastics more flexible-       Toxic effects: in animals, high oral doses of DEHP caused health effects mainly in liver and testes.-       IARC classification as a possible carcinogen
Cadmium -       Used in the manufacturing of nickel-cadmium batteries.-       Also used in industrial coatings and electroplating, in pigments and as a    plastic stabilizer in PVC plastics

(D. Kennedy, CLS 306 Mens and Women’s Health Lecture, July 12, 2012).

Properties of heat

The kinetic theory of gases explains that gases consist of many molecules or atoms moving randomly and continuously while frequently colliding with each other and with the walls of their containers. At higher temperatures, particles move faster and are able to make more interactions with one another (M. Facca, SGC 100 General Chemistry lecture, p.47, July 23, 2012). Heat in the form of thermal energy, provides the activation energy to break covalent bonds. In reference to plastics, heat is able to decompose polymers into monomers and/or toxic substances. In 2011, a study by Yang et al. reported that the majority of commercially available plastics when heated, released chemicals (xenoestrogens) with estrogenic activity (abbreviated EA) (Yang et al. 2011).

Polycarbonates & Bisphenol A (BPAs)

Polycarbonates are notoriously known for being added to plastics and used for food storage. Consider polycarbonates as an example when heated.

(The above image is a repeating chemical structure unit of polycarbonate made from Bisphenol A)

In an experiment conducted by Bair et al. in 2003, polycarbonate was placed in glass containers and heated to 85°C and 96% relative humidity (RH). The results produced brown surface crystals in a span of 30 days. Several studies were repeated at 70°C and lower with an RH of 96% and the results yet again produced a brown liquid coating (Bair et al., 2003). Measurement of the brown surface crystals showed that weighted 70% of the surface crystals were bisphenol A (BPA). The brown liquid coating in the subsequent experiments was composed of supercooled liquid BPA, BPA oligomers, and water (Bair et al., 2003).

Hydrolysis (degradation by water, often referred to as leaching) when occuring at high temperature will release bisphenol A. The breakdown of polycarbonate through hydrolysis is shown below:

1/n [OC(OC6H4)2CMe2]n + H2O → (HOC6H4)2CMe2 + CO2

(Howdeshell et al., 2003)

Therefore we can observe by heating foods at high temperatures, a chemical reaction in the form of hydrolysis will release BPA from polycarbonates and leach into foods (Howdeshell et al. 2003). The implications of this are profound as BPA is considered an endocrine disruptor and has mimicking properties of estrogen.

Various studies have shown that at high temperatures, leaching of BPA from polycarbonate can occur and have detrimental effects on animals and humans alike.

  • In 2008, BPA experiments by the National Institutes of Health’s National Toxicology Program on rats in continuous low-level exposure (dosages similar to human exposures), found an accumulation of BPA in the blood of mice and linked the chemical to precancerous lesions in the prostate and mammary glands and to early puberty in females rats  (Zuckerman et al. 2009). 

Implications & Recommendations

The decomposition of plastic when microwaved is not always apparent via the olfactory, gustatory and visual senses. However, heat is able to degrade the chemical bonds of plastics and release endocrine disruptors at low levels. Since 2010, Canada has classified BPA as inherently toxic and has banned its use in plastics (Environmental Health Association of Nova Scotia, 2007).

Be that as it may, polypropylene and polyethylene plastics may still contain BPA as polycarbonate is often combined with other plastics and there is no requirement for BPA content to be listed on plastic labels (Environmental Health Association of Nova Scotia, 2007). Milwaukee Journal Sentinel, in the fall of 2008, lab-tested 10 plastic food containers for microwave leaching, plastics Nos. 1, 2 and 5 had BPA leaching (Vasil, 2009). It is important to note that BPA-free plastics do not mean EA-free. Despite having being advertised as BPA-free, all the commercially available plastic products sampled leached xenoestrogens; in some cases, BPA-free products released more xenoestrogens than did those containing BPA (Yang et al. 2011).

Best Health Magazine recommends that when microwaving foods or drinks to avoid plastics and plastic wraps altogether. Dishwashing plastics (due to heat) can also boost leaching (Vasil, 2009). To be safe and to reduce the risk of chemical contamination, opt for ceramic or glassware alternatives when heating up food.

Some areas for future research: Leaching of plastics at room temperatures, Plastic leaching in dishwashers, PCBs, Phthalates, Cadmium, Polyesters and PolyVinyl Chloride.

Appendix A: Table of SPI Resin Identification Codes

Recycling Number


Polymer Name

Polymer Abbreviation


Polyethylene Terephthalate



High Density Polyethylene




Polyvinyl Chloride



Low Density Polyethylene









Other plastics, including acrylic, acrylonitrile butadiene styrene, fiberglass, nylon, polycarbonate, and polylactic acid



(The American Chemistry Council, 2012)


Bair HE, Falcone DR, Hellman MY, Johnson GE, Kelleher PG, et al. (2003) Hydrolysis of polycarbonate to yield BPA. Journal of Applied Polymer Science DOI: 10.1002/app.1981.070260603. Retrieved from: http://onlinelibrary.wiley.com/doi/10.1002/app.1981.070260603/abstract

Crisp TM, Clegg ED, Cooper RL, Wood WP, Anderson DG, Baetcke KP, Hoffmann JL, Morrow MS, Rodier DJ, Schaeffer JE, Touart LW, Zeeman MG, Patel YM (1998). “Environmental endocrine disruption: An effects assessment and analysis”. Environ. Health Perspectives 106 (Suppl 1): 11–56. PMC 1533291. PMID 9539004.

Environmental Health Association of Nova Scotia (2007). Microwave safe plastic – is it really safe? Retrieved from: http://www.environmentalhealth.ca/fall07microwave.html

Howdeshell KL, Peterman PH, Judy BM, Taylor JA, Orazio CE, et al. (2003). Bisphenol A Is Released from Used Polycarbonate Animal Cages into Water at Room Temperature. Environ Health Perspect 111(9): doi:10.1289/ehp.5993. Retrieved from: http://ehp03.niehs.nih.gov/article/citationList.action;jsessionid=632CCBECA57DCEE7DABD34CE24C609DA?articleURI=info%3Adoi%2F10.1289%2Fehp.5993

Michigan States University Department of Chemistry (2012). Polymers. Retrieved from: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/polymers.htm

National Institute of Environmental Sciences (2012), Endocrine Disruptors. Retrieved from: http://www.niehs.nih.gov/health/topics/agents/endocrine/

The American Chemistry Council (2012), Plastic Packaging Resins. Retrieved from: http://plastics.americanchemistry.com/Plastic-Resin-Codes-PDF

The Endocrine Society (June 16, 2009). Endocrine-Disrupting Chemicals. Retrieved from: http://www.endo-society.org/journals/ScientificStatements/upload/EDC_Scientific_Statement.pdf

The Plastics Industry Trade Association (2009). SPI Resin Identification Code – Guide to Correct Use. Retrieved from http://www.plasticsindustry.org/AboutPlastics/content.cfm?ItemNumber=823&navItemNumber=2144

Vasil A. Is it safe to microwave plastic? (2009). Best Health Magazine. Retrieved from: http://www.besthealthmag.ca/get-healthy/prevention/is-it-safe-to-microwave-plastic

Yang CZ, Yaniger SI, Jordan VC, Klein DJ, Bittner GD (2011). Most Plastic Products Release Estrogenic Chemicals: A Potential Health Problem That Can Be Solved. Environ Health Perspectives 119(7): doi:10.1289/ehp.1003220. Retrieved from: http://ehp03.niehs.nih.gov/article/citationList.action?articleURI=info%3Adoi%2F10.1289%2Fehp.1003220

Zuckerman D, Brown P, and Walls L. National Research Center for Women & Families. Are Bisphenol A (BPA) Plastic Products Safe for Infants and Children?; November 2009 Retrieved: http://www.center4research.org/2010/04/are-bisphenol-a-bpa-plastic-products-safe-for-infants-and-children/

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