Mylar bags thinner than 4 mils may be slightly transparent. Bags so thin would allow UV light to influence the freshness of food stored in the bag. Furthermore, thinner bags are more prone to puncture damage. However, they still provide an air-tight seal and are an excellent choice for smaller amounts of food that you might want to store for short to medium term.
For the best protection of your food stores over the long term, we recommend mylar food storage bags of 7 mils. They are completely opaque, allowing no UV light to enter and damage your food. Moreover, bags rated at 7 mils are much more rigid and resistant to puncture damage. At 7 mils, a 5-gallon mylar bag will hold about 35lbs of flour and keep it safe from contamination for up to 30 years.