It is not completely true, but if we keep it short then yes it is bad for health for various reasons. Many people think that popcorn is junk food but it is not because it is a whole grain that is rich in fiber, low in calories, and high antioxidants. Popcorn has a high nutrient value that helps us from hypertension and maintain our weight. But Microwave popcorn bags contain some chemicals which can lead to cancer, the gas released from it is also bad for health as it can cause popcorn lung disease, there are other reasons to like getting unhealthy fat from it, high levels of sodium in it etc.
So it is always better to make popcorn healthy in a traditional way.
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