Introduction
When it comes to reheating leftovers, many people glance at the bottom of a container to check the small recycling symbol before tossing it into the microwave. One of the symbols that tends to cause confusion is the triangle with the number 6 inside, which stands for polystyrene. With so many takeout containers, foam trays, and disposable cups made from this material, a very common question comes up: Can You Microwave Number 6 Plastic?
The short answer is that in most cases, no, you should not microwave number 6 plastic. However, the full answer depends on the form of the plastic, whether it has a microwave-safe label, what type of food you are reheating, and how long it is exposed to heat. Understanding the difference between rigid polystyrene and expanded polystyrene, as well as how heat can affect this material, is the key to making a safer choice for you and your household.
This guide breaks down everything you need to know about number 6 plastic and microwaving. From identifying it correctly, to understanding the potential risks, to practical safety tips and safer alternatives, every section is designed to help you make an informed decision without relying on guesswork.
Table of Contents
- Introduction
- What Is Number 6 Plastic?
- Forms of Number 6 Plastic
- How Microwaves Affect Plastics
- Can You Microwave Number 6 Plastic?
- Understanding Microwave-Safe Labels
- Potential Risks of Microwaving Number 6 Plastic
- Step-by-Step Guide for Safe Reheating
- Number 6 Plastic vs Other Plastics in the Microwave
- Safer Alternatives to Microwaving Number 6 Plastic
- Tips and Best Practices
- Common Mistakes to Avoid
- Frequently Asked Questions
- Conclusion
What Is Number 6 Plastic?
Number 6 plastic refers to polystyrene, often abbreviated as PS. The resin identification code on the bottom of an item identifies the type of plastic resin used to make it, not automatically that it is safe for every use. Polystyrene is a lightweight, rigid, or foamed plastic that has been used in a wide variety of packaging due to its ability to insulate, its low weight, and its affordability.
In its solid form, polystyrene can look clear or colored and is used for things like certain disposable cutlery, lids, or rigid containers. In its foamed form, it is most recognizable as expanded polystyrene, which is used for meat trays, egg cartons, takeaway clamshells, and many insulated cups. Many people refer to expanded polystyrene by a common brand name, but the material itself is still number 6 plastic.
It is important to note that the presence of the number 6 alone does not mean a container is designed for high heat. Polystyrene has a relatively low heat tolerance compared to materials like glass or ceramic, and this is the main reason why caution is needed when considering microwaving it.
Forms of Number 6 Plastic
Not all number 6 plastic looks the same, and the form it takes plays a big role in how it responds to microwave heat.
Rigid Polystyrene
Rigid polystyrene is solid and often has a slightly glossy finish. You may see it in some clear or opaque disposable items. It is stiffer than expanded forms, but it can still soften when exposed to high temperatures for long periods. Even in rigid form, most of these items are intended for cold or room-temperature use rather than direct, prolonged heating in a microwave.
Expanded Polystyrene (EPS)
Expanded polystyrene is made up of tiny beads of polystyrene filled with air. This air content is what gives it its excellent insulating properties. However, that same structure makes it very prone to warping, shrinking, or melting under intense heat. Foam trays from the grocery store, many hot-food foam containers, and insulated foam cups are typically made from this form. For the vast majority of these, microwaving is not recommended.
Extruded Polystyrene
Extruded polystyrene is less common for single-use food containers and is more often seen in building insulation. It is rarely the type of number 6 you would find as a leftover food container, so for this specific question, rigid and expanded polystyrene are the forms that matter most.
How Microwaves Affect Plastics
To understand why the question Can You Microwave Number 6 Plastic? has such a cautious answer, it helps to know the basics of how microwave heating works. Microwaves heat food by causing water molecules to vibrate, which generates heat throughout the food. The container itself is not directly “cooked” by the waves in the same way, but it can absorb heat that is transferred from the hot food or from uneven hot spots.
Different plastics react differently to this transferred heat. Some plastics are formulated to stay stable up to certain temperatures. Others begin to soften at relatively low temperatures. Polystyrene, especially in foam form, has a low softening point. When it reaches that point, it can lose its shape, become tacky, or start to melt.
Another concern with heating certain plastics is the potential for chemicals to move from the plastic into food. This is often referred to as migration. Factors that increase the chance of migration include high temperatures, long heating times, and foods that are high in fat or oil. Fatty and oily foods tend to get very hot in the microwave and can transfer more heat to the container than water-based foods would.
Can You Microwave Number 6 Plastic?
For the overwhelming majority of number 6 plastic containers, the answer is no. Most takeout foam boxes, meat trays, egg cartons, and disposable foam cups are not made with the intent of going in a microwave. Heating them can cause them to warp, collapse, or melt, and it is not considered a safe practice.
There are very rare exceptions. A small number of rigid polystyrene containers may be specifically manufactured and labeled as microwave safe. If a container is explicitly marked with a microwave-safe symbol and states it is suitable for microwave use, then it may be intended for that purpose by the manufacturer. Even then, you should only follow the exact instructions provided for that specific item.
Never assume that the recycling number alone means microwave safe. The triangle is for recycling identification, not a universal safety rating for heating. Also, do not rely on age, thickness, or how sturdy the container feels. A thick foam clamshell can still soften and deform under the heat of oily leftovers.
In short, unless you can see a clear “Microwave Safe” marking directly on that exact number 6 container, do not microwave it.
Understanding Microwave-Safe Labels
Many people look for a symbol to quickly decide if a container is okay for the microwave. Knowing what to look for can help prevent avoidable mistakes.
Common Symbols to Know
A true microwave-safe label will usually say the words “Microwave Safe” in text. Some containers also use a symbol that looks like a dish with wavy lines above it. Either form is meant to indicate that the manufacturer has tested that specific container for microwave use under stated conditions.
What the Number Does and Does Not Mean
The resin code (1 through 7) tells you what type of resin the item is made from. It does not tell you if it is heat-resistant, oven-safe, or microwave-safe. Two items can both be number 6, yet one may be a thin foam tray never intended for heating, while another rare rigid version could be made differently. Only the explicit microwave-safe marking applies to that item.
Read the Full Instructions
If a container is labeled microwave safe, pay attention to any limits. Some may say to remove the lid, avoid heating for long periods, or only use for certain types of food. Following those directions matters just as much as the label itself.
Potential Risks of Microwaving Number 6 Plastic
While not every use will result in visible damage, there are practical reasons to avoid microwaving most number 6 plastic. It is better to focus on known, avoidable risks than to guess.
Melting and Deformation
The most obvious risk is physical. Polystyrene softens at a relatively low temperature. When heated, foam containers can sag, shrink, or partially melt. Rigid versions can warp out of shape. Melted plastic can stick to food, be difficult to remove safely, and ruin the container entirely.
Leaching Concerns
When plastics are heated, there is a greater chance of chemical migration from the plastic into the food. With polystyrene, the compound of interest is styrene. The amount can depend heavily on temperature, time, and food type. Fatty foods like pizza, fried items, or foods with oil tend to reach higher temperatures and can increase contact between hot food and the container. While research continues to evaluate exposure levels, the most practical and widely recommended approach is to minimize unnecessary heating of polystyrene food containers.
Hot Spots and Uneven Heating
Microwaves create hot spots in food. These spots can become much hotter than the rest of the meal and can cause localized melting of the container even if other areas seem fine. A small melted patch is easy to miss and can transfer plastic to that portion of food.
Lid and Seal Issues
Many number 6 containers come with separate lids made from a different plastic. Even if the base is somehow considered, the lid may not be. Heating a sealed foam container can also trap steam, increasing pressure and the chance of warping or sudden softening.
Step-by-Step Guide for Safe Reheating
If your goal is to reheat food safely, the best way to avoid issues with number 6 plastic is to transfer your food first. Use this simple, practical approach every time.
- Check the container. Look at the bottom for the recycling symbol and number. If you see a 6, treat it with extra caution.
- Look for a clear label. Only consider microwaving if the exact container has the words “Microwave Safe” printed on it. Do not guess based on look alone.
- Remove lids and covers. If you ever decide to use a microwave-safe item, remove any plastic lid unless the instructions specifically say to leave it on with proper ventilation.
- Transfer when in doubt. If there is no clear microwave-safe label, empty your food into a microwave-safe dish made of glass, ceramic, or a plastic clearly marked as microwave safe.
- Use appropriate cookware. Choose a wide, shallow dish to promote more even heating and avoid long cook times.
- Heat in short intervals. Reheat in 30 to 60 second intervals, stirring in between to spread heat and reduce hot spots.
- Check temperature carefully. After heating, test a small bite away from any very hot spots. Use caution when removing the dish, as it may be hot.
- Inspect after use. If you ever accidentally heated a non-labeled number 6 container and notice warping, melting, or a strange texture, discard both the container and any food that came into direct contact with the melted areas.
Number 6 Plastic vs Other Plastics in the Microwave
Seeing how number 6 stacks up against other common resin codes can make the answer even clearer. The table below is a general, practical comparison for typical food containers you may come across. Keep in mind that labels always override the number.
| Resin Code | Common Name | Typical Use | General Microwave Outlook | Notes |
|---|---|---|---|---|
| #1 | PET or PETE | Water bottles, some clear trays | Usually not recommended for microwaving | Mostly made for single use and not designed for high heat |
| #2 | HDPE | Milk jugs, some thicker containers | Generally not intended for microwave use | More stable in some forms, but most food containers of this type are not microwave-safe |
| #3 | PVC | Some wraps and older containers | Not recommended | Rarely used for modern microwaveable food trays |
| #4 | LDPE | Some bags, flexible lids | Usually not recommended | Flexible and can melt or warp with direct heat |
| #5 | PP (Polypropylene) | Yogurt cups, some reusable containers, many microwave-safe takeout lids | Often microwave-safe when clearly labeled | One of the more common resins used in containers designed for reheating |
| #6 | PS (Polystyrene) | Foam cups, foam trays, egg cartons, some rigid disposables | Mostly not recommended | Only a rare, clearly labeled rigid version may be intended for microwave use |
| #7 | Other | Varied plastics, some blends | Varies greatly | Must be checked case by case for a microwave-safe label |
As this comparison shows, number 5 is the one most often found in containers actually designed with reheating in mind. Number 6 sits on the opposite end for most everyday foam and disposable packaging.
Safer Alternatives to Microwaving Number 6 Plastic
The easiest and safest solution is to skip microwaving number 6 plastic altogether and use a material built for it. These options are widely available and do not require guessing.
Glass Dishes
Glass is non-reactive, holds up well to microwave heat, and does not soften or warp from transferred heat. Many glass containers come with lids made from other materials, so just be sure to use a lid that is also marked microwave safe or leave it slightly vented.
Ceramic Plates and Bowls
Plain ceramic without metallic paint, trim, or decals is a great choice for reheating. It distributes heat reasonably well and is easy to clean. Avoid any dish with gold, silver, or other metallic accents, as those are not suitable for the microwave.
Microwave-Safe Plastic Containers
If you prefer plastic, use only containers that are clearly labeled Microwave Safe. Look for those made with resins commonly used for that purpose and always follow any usage notes. Even with labeled plastic, avoid super-long heating times and very oily foods when possible.
Microwave-Safe Paper Products
Some plain, uncoated paper plates or microwave-safe paper bowls are designed for short reheating times. Always check the product labeling, avoid anything with wax coatings, plastic linings not meant for heating, or metallic prints.
Tips and Best Practices
Keeping a few simple habits in mind will go a long way when dealing with the question of Can You Microwave Number 6 Plastic?
- Default to transfer. Make transferring leftovers to glass or ceramic your automatic habit. It only takes a few seconds and removes almost all of the related concerns.
- Never trust foam blindly. Expanded polystyrene looks harmless, but it is the form most likely to warp or melt under microwave heat.
- Watch for fat content. If a food is very greasy or oily, it will get hotter faster and transfer more heat to any nearby plastic. This applies even to containers that feel sturdy.
- Vent covers properly. Trapped steam builds pressure. If using any covered container, vent it according to instructions.
- Skip old or damaged containers. Scratches, cracks, or previous warping can make any plastic less stable when exposed to heat.
- Read, don’t guess. Treat “Microwave Safe” as a requirement, not a bonus. If it’s missing, leave it out of the microwave.
- Think beyond reheating. Also avoid using number 6 containers for cooking from scratch, defrosting for long periods, or any method that would expose them to sustained high heat.
- Keep kids and quick meals in mind. Rushed reheating is when most people make this exact mistake. A second dish is worth the extra moment.
Common Mistakes to Avoid
Many of these mistakes are incredibly common, yet easy to correct once you know them.
Assuming Foam Is Microwave Safe
Just because some foam containers were used to hold hot food from a store does not mean they are safe to microwave. Holding hot food during transport is very different from being heated directly by microwave energy with no barrier.
Microwaving With the Lid On
A tight-fitting foam lid can trap steam and increase the chance of warping. It can also block proper venting. Even if the base were somehow okay, the lid is often a different material and usually not intended for heating.
Heating Greasy Leftovers in the Original Container
Foods like french fries, wings, or anything cooked with oil can reach very high surface temperatures. These hot, greasy spots are the most likely to cause localized melting of number 6 plastic.
Relying on the Recycling Number
This is the single biggest point of confusion. The number 6 is for sorting recyclables, not a microwave approval code. Always look for words on the container, not just the triangle.
Trying to “Test” It
Some people try to microwave for a few seconds to see what happens. By the time you notice warping or softening, the container has already been exposed to heat. There is no safe at-home test to rule out all concerns, so it is not worth doing.
Frequently Asked Questions
Is any number 6 plastic actually microwave safe?
Only in rare cases. A very small number of rigid polystyrene items are made and clearly labeled Microwave Safe by their manufacturer. The vast majority of number 6 plastic, especially expanded foam, is not. If you do not see that exact label on that container, do not microwave it.
Is Styrofoam the same as number 6 plastic?
In most food packaging, yes. “Styrofoam” is a common term people use for expanded polystyrene, which carries the resin code 6. There are different types of polystyrene products, but when referring to foam takeout cups and trays, they are typically number 6.
What happens if I accidentally microwave number 6 plastic for a short time?
It depends on the container, food, and time. You might notice no visible change, or you might see slight warping. With greasy or very hot foods, melting can happen faster. If you see any melting, softening, or the container sticking to food, discard the container and do not eat any food that was directly in contact with the melted plastic.
Are number 6 egg cartons microwave safe?
No, typical paperboard-like or foam number 6 egg cartons are not intended for microwaving. They are designed for storing eggs, not for reheating food or cooking. Always transfer eggs or any food to an appropriate microwave-safe dish.
Can I microwave number 6 plastic if I put a paper towel over it?
No. A paper towel does not make a non-microwave-safe container safe. It may absorb some splatter, but it will not stop the plastic from softening or from transferring heat in a way that increases the chance of deformation or migration.
Is it safer to microwave with the number 6 container off-center or on low power?
Lowering power or changing placement does not make a non-labeled number 6 container safe for microwave use. Heat can still build up, especially from hot food. The material itself still has a low heat tolerance, so this approach is not a reliable way to avoid the risks.
Which plastic numbers are usually okay for the microwave when labeled?
When clearly labeled Microwave Safe, number 5 (polypropylene) is the most common for reusable or reheatable containers. Some number 7 items may also be designed for it, but those must be checked on a case-by-case basis. Number 6 should be assumed unsafe unless explicitly stated otherwise.
Why do some restaurants give hot food in foam number 6 containers?
Expanded polystyrene is used mainly for insulation. It keeps hot food warm and cold food cold while being lightweight. That is a different purpose than being designed to go back into a microwave for reheating.
Conclusion
So, Can You Microwave Number 6 Plastic? For almost every foam tray, cup, or clamshell you will come across, the answer is no. The only realistic exception is a rare, rigid number 6 container that has a clear and explicit “Microwave Safe” label from the manufacturer. Without that label, you should never assume it is okay.
Polystyrene simply does not have the heat resistance needed for most microwave reheating, especially when dealing with oily or high-temperature foods. The risks of warping, melting, and unnecessary exposure are easy to avoid with one simple habit: transfer your food to glass, ceramic, or a plastic container that is clearly marked as microwave safe.
By checking labels, using the right cookware, and following the safe reheating steps in this guide, you can reheat your meals with far less worry. When in doubt, skip the number 6 plastic and reach for a safer alternative. That small choice is the most practical and reliable way to protect both your food and your peace of mind.