For busy urban people, takeaway food is a godsend. But before you pop that plastic container into the microwave oven to warm the food up, there are safety issues to consider.

Not all take-out food containers are safe for microwave use. Plastic containers that are not microwave safe could melt and may leach chemicals into food. And styrofoam containers, for instance, cannot withstand high heat and are unsuitable for microwave use, said Dr Lee Mun Wai, senior manager (packaging innovation) at the Food Innovation and Resource Centre at Singapore Polytechnic.

To err on the safe side, always use microwave-safe containers.

Containers without any microwave-safe labels are not considered unsafe, but the user might not be assured that the containers have been evaluated and qualify for microwave heating applications, said Dr Lee.

Additives or coatings added into the packaging to enhance other properties might render the container unsuitable for microwave use, he said

Apart from a microwave-safe label, it is also possible to tell if a container is safe for microwave reheating by its recycling code, said Dr Wong Seng Weng, medical director of The Cancer Centre, a Singapore Medical Group clinic.

This code resembles a triangle formed by three arrows with a number, from 1 to 7, in the centre.

Generally, only code 5 is able to withstand the heat from microwave reheating.

Here, we look at why some common food containers and kitchen items are suitable for use in the microwave while others are not.

When plastic containers are labelled as microwave safe, it means that only very minuscule amounts of plasticisers (within the limits of human health safety) are released into the food when subjected to heat, said Dr Wong, a medical oncologist.

“One has to be careful because the covers of many food containers are not made with the same material,” he said.

“If the container is labelled code 5 and the code is unspecified on the cover, remove the cover before reheating.”

Whether plasticisers are indeed carcinogenic is still controversial and inconclusive, said Dr Wong.

“There is a lot of talk on the Internet about dioxin (a known carcinogen) leaking into food from plastic containers during microwave reheating,” he said.

“But plastic generally does not contain dioxin and, hence, such fearmongering is misleading.”

Some plastics, however, contain bisphenol-A (BPA). “While the evidence is not conclusive, there is some concern that BPA may increase the risk of breast and prostate cancer,” said Dr Wong.

Polycarbonate plastic (code 7) is more likely to contain BPA, he said.

Plastic wrap is not heated by microwave energy but it can warp or even melt if it comes into contact with extremely hot food.

Most manufacturers recommend leaving an inch of air space between the food and the wrap, said the American Chemistry Council.

As with other plastic packaging, use only microwave-safe plastic wrap. You can also use wax paper or white paper towels.

The council also said that many plastic containers in which foods are sold, such as those used for butter and cottage cheese, are designed for cold storage and not intended for cooking or reheating. And most plastic trays provided with pre-packaged meals are intended for one-time use only, it said.

These are suitable for microwave ovens as they are able to withstand high temperatures.

However, if the glass or ceramic wares are not certified microwave safe, there are safety issues to consider as the presence of micro-bubbles in a glass container might cause it to crack, for instance, said Dr Lee.

Do not use a glass bowl with a metal rim in a microwave oven.

These should not be used in a microwave oven, as metal containers tend to reflect the microwaves, which prevents them from penetrating the container to reach the food, said Dr Lee. The food may be unevenly cooked.

Furthermore, metals are known to possess free electrons, which can possibly damage the oven. In extreme incidences, holes can be created in the metallic wall of the microwave oven due to the high heat, said Dr Lee.

Dr Wong said that the inner layer of many canned food containers are lined with BPA-containing resin. “Hence, heating canned food in its original container may not be safe, either.”

Melamine ware is for serving and should not be used in a microwave, said Dr Lee.

He pointed out that the United States Food and Drug Administration has advised against the use of such tableware in a microwave oven as there is a risk of chemicals migrating into food, particularly high-acid food, when heated to temperatures beyond 71 deg C.

This article was originally published in Mind Your Body on June 4 2015.