If it doesn't clearly say "microwave-safe", it probably isn't. Even if marked as "microwavable", the plastic should not be allowed to come into direct contact the food during the microwave cooking process.
There is a "microwavable" symbol, but with a few exceptions, there are no "microwave safe symbols" in common use. 
Obviously, plastics vary in their chemical composition. What they have in common is that they are formed by chemical reaction. If used with food in a microwave,
The important thing is to identify those plastics which are microwave-safe.
- they must not further react;
- they must not decompose into the food; and
- they must not leach out components into the food.
There probably are plastics which are safe for microwave use. The epoxy material used in microwave construction is an example of this (although generally not in contact with the food). Unfortunately, the available information on food safety is scarce and often contradictory:
NSF, which certifies food-safe items, states:But they seem to have gotten it wrong:
- Only use cookware that is specially manufactured for use in the microwave oven. Glass, ceramic containers, and all plastics should be labeled for microwave oven use.
- Never use plastic storage containers such as margarine tubs, take-out containers, whipped topping bowls, and other one-time use containers for cooking or reheating food in your microwave. These containers are not heat resistant and can melt, possibly leaching harmful chemicals into your food.
- Microwave plastic wraps, wax paper, cooking bags, parchment paper, and white microwave-safe paper towels should be safe to use. Never use thin plastic storage bags, brown paper or plastic grocery bags, or aluminum foil in the microwave. Never microwave unidentified plastic wrap with food because some are PVC.As to "brown paper bags", some people advise using kraft paper as the only safe disposable material for microwave use. It is possible that brown paper bags could present a fire hazard in the microwave under some circumstances, so NSF may be right on that one.
- Glass does not cause food safety problems in a microwave, and is generally safe for use in a microwave. This includes such items as leaded glass. Glass is rarely labeled.I have personally heated water in 24% lead glass manufactured by Crystal d'Arques, with no unusual effect. (Leaded glass should not be used for long term food storage and should not be used for storing acidic liquids, but it does not otherwise present a health hazard.)
- Ceramic is generally not labeled. There are some ceramic dishes which cause problems, either because of metal or because the glazing will become hot, but these do not decompose into food. Ceramic does not present food safety problems.Examples of coatings which can heat ceramic include tin oxide, which is intentionally used on some ceramic microwavable saute pans to apply surface heat to food. 
- Many white paper towels are fabricated with plastics, and are not microwave safe.
- Glass and ceramic are the preferred materials for use in a microwave oven.
- They forgot to mention to keep plastics separate from the foodstuff.
- They totally ignored paper plates. Plain paper is safe, but plastic coated paper is generally not microwave-safe.
- They fail to mention bisphenol A (BPA), diisononylphthalate (DINP), and other plasticizers.
Wisegeek.com has an article describing issues and what to look for, and describes an international symbol for microwave-safe plastics. 
and Identifying BS
The plastics industry likes to point to an alleged story about plastics forming dioxins in a microwave. This is a totally bogus issue. For one thing, there is no reason for plastic containers to contain dioxin unless the purpose is to store dioxin. Dioxins typically form at temperatures above 370°C. The dioxin issue is of course easy to refute, which appears to be the reason it is raised in the first place.
The dioxin issue has the result of obfuscating such issues as plastics decomposing in food, plastics reacting, and release of toxic elements which are in some plastics. The plastic industry should be educating consumers what plastics to use and not use in a microwave oven. Bringing up a "dioxin" issue makes it more difficult to identify which, if any, plastics are safe for microwave use.Significantly, most of the "dioxin" stories specifically avoid mentioning the issues of bisphenol A (BPA) and the related issue of diisononylphthalate (DINP). If the article addresses toxins leaching from plastic, it should not avoid the specific issue of bisphenol A and other plasticizers!If an article claims to address plastic safety, but instead focuses on the dioxin issue, then consider it bogus!
Recycling symbols on the container indicate the type of plastic, but are not definitive of microwave safety. (They also don't indicate that the plastic is recyclable.) Symbols that identify the type of plastic in a container do not themselves indicate microwaving suitability. Some packages made of polyethylene terephthalate (PET, type 1), for example, are fabricated for high heat resistance, while others are not.
The "recycling types" are at least useful for determining which plastics to avoid in the microwave.(These "recycling types" are also known as "SPI numbers", named after the North American Society of the Plastics Industry, which developed the codes.)
Type 1 - polyethylene terephthalate (PET)May be safe if marked "microwave safe", although some recommend against food contact when microwaving. Type 2 - high density polyethylene (HDPE)May be safe, although some recommend against food contact when microwaving. Type 3 - PVC, polyvinyl chloride, vinylDo not use in microwave. Also, PVC often contains bisphenol A (BPA). Since this material is used for some plastic wrap, unidentified plastic wrap should never be used in a microwave with food! Type 4 - low density polyethylene (LDPE)(mixed commentary regarding microwave safety) Type 5 - polypropyleneMay be safe, although some recommend against food contact when microwaving. "Type 5" are the most commonly labeled "microwave safe". Despite this, I have observed "Type 5" containers with partially dissolved surfaces, apparently from microwave use. Type 6 - polystyrene, styrene, polystyrene foamNot heat stable. Do not use in microwave. Do not microwave food in a styrofoam  container! In addition to not being heat stable, polystyrene is a potential human carcinogen and usually contains bisphenol A (BPA). Type 7 - polycarbonate; "other" (can contain bisphenol A (BPA); most polycarbonate contains bisphenol A)Do not use in microwave. (Note: Polycarbonate nursing bottles which have been boiled or washed more than 20 times or are badly scratched should be thrown out.) paper - food safe but Avoid using paper coated with plastic in the microwave.(Some types of paper may catch on fire under some circumstances. Some sources suggest avoiding using newsprint. Paper which is not food grade may also include toxic inks.)
Never use plastic storage bags, grocery bags, newspapers, or aluminum foil in the microwave.
If you use plastic wrap in a microwave, read the label -- use only materials marked "safe for microwave". If it doesn't say "microwave safe" there's no reason to presume it is! Most wraps also require a one-inch or greater space (2.5 cm) between the food and the wrap.
|Photo of Clare Nelson, who conducted research on concerning microwaving plastics with food. According to the American Plastics Council, she was granted an award "... based on her systematic approach to exploring a question". related to migration of DEHA and other plastics additives into food.|
I have never seen a certification of a plastic as "microwave safe" by the FDA (US Food and Drug Administration) or similar authority. The American Chemical Counsel mentions stringent FDA safety standards (see below), but there is no microwave testing procedure under the FDA for food containers.To the contrary, see below for an example of food in a polystyrene container, with microwave instructions.
The FDA points out that most plastic wraps labeled "safe for microwave" also have labels indicating that there should be a one-inch or greater space (2.5 cm) between the plastic and the food during microwave heating.(Wikipedia's article on "Safe Sex" included a curious comment that "microwave-safe" wrap may be ineffective as a barrier for virus-sized particles when engaging in oral sex. This is probably incorrect, as there is no structural difference relevant to protection from STDs. In either case, do not place your partner in the microwave.)
The Canadian Food Inspection Agency is more direct about microwaving in plastics:
"Remove food from plastic wrap, freezer cartons, and/or styrofoam trays before defrosting and cooking. They are not heat stable and could leach hazardous compounds from the container or plastic wrap to the food."Food Inspection Agency website (Most of the article regards food safety issues unrelated to chemical food contamination.)
This plasticizer is leached out by heating in microwave ovens.
More information Here. (Needless to say, plastic nursing bottles and non-glass accessories should not be microwaved.)
- NSF www.nsf.org/consumer/food_safety/fsafety_cooking.asp?program=FoodSaf
- "Many of the foam or plastic trays and wraps are not heat stable and may melt at the high temperatures produced by microwaves."
- American Chemistry Council - Note that they specify that not all plastics are safe for microwave use, but then comment about FDA safety standards.
- "Microwave cookingFor the full textual context, see American Chemistry Council - Keeping a Safer Kitchen
... plastics work well with microwave ovens ... However ... it is easy to get confused about which are safe for microwave use.
If there are no such instructions, the packaging should be discarded ...
... proper use is important, as plastic wrap can melt if it is allowed to come into contact with extremely hot food. ... the wrap should cover the dish without touching the food.
All plastics intended for use with food must meet stringent U.S. Food and Drug Administration (FDA) safety standards before they are allowed on the market. ... Choosing to microwave with a plastic item not labeled for that purpose is not necessarily unsafe, but there is no assurance of knowing that the item was tested and evaluated for such use.
(Doesn't that define it as "unsafe"?)
Do not use plastic containers for cooking unless they are intended for that purpose. ...
Symbols that identify the type of plastic in a container do not themselves indicate microwaving suitability. ..."
(comments on dioxin deleted)
- "Plastic containers, unless they're deemed microwave-safe, are a definite no-no."
"In other words, those handy little tubs that once held such victuals as yogurt, margarine or pudding should never get zapped behind the microwave door - and the same is true of take-out food containers made of styrofoam, a substance that isn't heat-stable."
"Many plastic wraps contain a plasticizer called diethylhexyl adipate (DEHA), which gives the products their softness and clinginess ... DEHA can leach into food during microwaving and is particularly absorbed by fatty foods such as meat and cheese." 
- Law Firm of Keller and Heckman LLP
- "no single regulation explicitly covers microwave applications. Neither do the Conditions of Use outlined at 21 CFR 176.170(c) expressly address microwave uses."
- (blog apparently directed to bisphenol-A)
- ... but
- A quick look at the supermarket shows there is substantially no FDA oversight:Looking in the soups section, I picked up a container of Maruchan noodles (also here). This was a polystyrene (type "6") cup (presumably with the gourmet MSG-and-noodles mixture inside). The cardboard cover had cooking instructions, with the following microwave instructions:I didn't have to open the box -- one was already half-opened on the shelf for me. Inside, printed on the tear-off lid:
"Microwave directions: See lid for details."This was followed by a warning that heated food is hot. There was absolutely no warning concerning chemical food safety. There was no warning regarding contamination of the food by heating the polystyrene container. This was not a third-world import. A look at the label showed, Maruchan, Irvine, California. (It is a division of Toyo Suisan, a Japanese conglomerate.) There was no indication why the Maruchan company was so circumspect about its microwave instructions, yet completely failed to warn about chemical safety.
"Because of variances in power of microwave ovens, it is best to boil water in a separate container"
Be careful with containers promoted as "microwavable".For example, Tupperware promotes some, but not all of their containers as microwavable. Their website describes, "
Tupperware offers a wide selection of microwave safe plastics". More interesting, "
Container is microwave safe (remove seals)". As nearly as I can determine, the lid is the seal! (I presume these lids are clearly marked "not microwave safe" in order to avoid this sort of error. The Tupperware website did not indicate the plastics used for either the containers or the "seals".) Obviously, if you use these things, you should find some sort of microwave-safe lid.
I hadn't figured out which, if any, plastics are safe for cooking. I just use ceramic or glass, and cover it with a glass cover or another dish.
The obvious answer — use ceramic dishes — is sometimes not available. Other alternatives are oversized coffee mugs and uncoated paper.
Fortunately, in most cases, it is convenient to bring a reusable container. It is possible to locate glass storage containers with plastic storage lids. (The plastic lid can be used for carrying the food in the container, and removed for cooking.) Other portable food containers are also available.
General Information on Plastics and Xenoestrogens:(all links open new pages)
- List of Carcinogenic Substances from Woman-Health.org
The 4th item mentions organochlorines, referencing the tech article below.
- Breast Cancer Fund
- University of Cincinatti Academic Health Center
- drKoop.com article
- Di(2-ethylhexyl)phthalate (DEHP) from the US Dept. Health and Human Services Agency for Toxic Substances & Disease Registry
- Health Effects of Plasticizers Used in Medical Devices and Toys - Medscape Today
- Plastics, Plasticizers and Phthalates from Aerias AQS IAQ Resource Center
- Joachim Payne, Martin Scholze, and Andreas Kortenkamp "Mixtures of Four Organochlorines Enhance Human Breast Cancer Cell Proliferation", Environmental Health Perspectives, v.109, no.4 (Apr.2004)
- This is definitely technical reading, but the implications are fairly clear.
- summary of "The Health Effects of DINP diisononylphthalate"
- (full article)www.net.org/health/products/toyscience.vtml
- I'll add more as I come across them. Some of my current work involves cancer diagnosis, and so I expect to see a few more tech journal articles. Much of the data is not on the web. For example of the 51 citations in the Payne article above, the only URL renders a 404 in German ("Datei nicht gefunden")
Other References - some differences, but all urge caution and urge identifying the particular plastic is labeled as safe before using:
- www.cookingforengineers.com "Microwave Safe Containers" (article)
- Harvard Medical School
- LECOM (Lake Erie College of Osteopathic Medicine)
- Don't Bus Disposables - suggests only bussing re-usable items.
If an article purports to address chemical food safety, consider whether it only discusses non-issues (e.g., dioxins). If it fails to address meaningful issues, that should tell you something.
FOOTNOTES:^ ^ Wisegeek describes an international symbol as a "square with a small dish at the bottom and 5 rows of waves, above the dish, stretching across the square." (article) This symbol does not appear to be in common use.
^ The most common tin oxide coated containers are Corningware browning dishes (also badge engineered as Amana) are out of production as of 2000, and the Pyroceram cookware factory in Charleroi, PA where these were made was shut down in 2001. These browning dishes are widely available on eBay. Note that some of the listings for "browning pans" are for aluminum-bottomed pans that are not microwave dishes.
^ ^ "Styrofoam" is a trademark of Dow Chemical, but is often used generically (especially in North America) to describe polystyrene. The primary significance of this is that it appears that Dow Chemical does not market "Styrofoam" cups, so "Styrofoam" cups do not really exist. If only that were true! (There's also an ambiguity about whether "Styrofoam" is expanded polystyrene foam or extruded polystyrene foam, a distinction that is probably of interest primarily to plastic manufacturing engineers.)
^ Carol Zweep, a senior research scientist at the University of Guelph's Food Technology Centre.
first posted 4-Nov-06; rev 16-Jan-14 This page copyright 2006, Stan