The main concern about cooking in plastic bags involve leaching of potentially harmful chemicals, such as BPA (Bisphenol-A) and Phthalates from the bag into the food. Food grade plastic bags, certified as suitable for cooking by their manufacturer, are safe to use.
Not all plastics are suitable for sous vide. Avoid the ones containing Polyvinyl Chloride (PVC) as they contain plasticizers. Suitable ones are bags made from high-density Polyethylene (HDPE) or low density Polyethylene (LDPE) - most sous vide or food-safe freezer bags fit this criteria.
In relation to resealable bags such as ziploc, sous vide practitioners often go for the heavy duty ones suitable for freezer storage. They are BPA-free and often used for sous vide when prolonged storage is not needed. We leave it to experts to comment on this issue:
Harold McGee, author and prominent food science expert commented in an article in the New York Times (11 August 2008):
''Heavy-duty Ziploc bags are made from Polyethylene (PE) and are approved for contact with hot foods. True sous vide cooking involves vacuum-packing the food, which zipping a bag won't do for you. But you can certainly use the bag to immerse food in a water bath whose temperature you control carefully. It can be hard to squeeze out all the air, so the bags tend to float and heat unevenly unless you weigh them down. Sous vide cooking generally involves water temperatures between 120°F - 180°F (49°C - 82°C), which the heavy-duty bags can take.''
Nathan Myhrvold, author of Modernist Cuisine also advised users to:
''Avoid bags containing Polyvinyl Chloride (PVC). Ziploc bags for home cooks are a safe alternative because they are made from Polyethylene (PE), which does not break down at the low temperatures of sous vide cooking and are BPA-free.''