It's no secret that scientists have determined that plastics are harmful to the environment, people, and to animals. Certain types of plastics are also extremely bad for keepsakes, photos, textiles, or any item you're thinking about keeping for the future. Plastic has become our catch-all for storing, well, literally everything. Whether its food, your winter clothes, or those boxes of photos in your basement, our society has been conditioned to embrace this magical material. And while it's cheap and convenient, it's not ideal for your keepsakes for a multitude of reasons.
Here's a short breakdown of plastics based on their recycle code:
In terms of museums and keepsakes, you basically want to avoid any kind of plastic in general, as most off-gas over time, releasing harmful chemicals which can then affect your items. Yes, they keep out water, bugs, dust, and a whole manner of other nasty things, but they can also contribute to unsightly staining, brittleness, cracking, or even disintegration over time. If you're items are stored in a plastic enclosure as it starts to off-gas, those chemicals have no where to go but into your items, which then speeds up deterioration.
Essentially, avoid anything with the recycle code 1 (PETE or PET), 3 (PVC), 6 (PS), or 7 (other) - these are especially harmful to both your objects and your health.
For museum objects, or ephemera/photographs/items you'd like to keep for posterity, one plastic - recycle code 5 - polypropylene (PP) - is chemically inert and won't transfer hazardous materials to your objects. Polypropylene is mainly used for photographic enclosures in museums, but is extremely expensive.
Finding polypropylene enclosures (like large tupperware bin storage you can put under your bed/stack/etc), has become increasingly hard in regular retail markets (such as Target, Walmart, Lowes, Home Depot). Most enclosures will be made of PET or won't be marked at all. If you want a polypropylene box, you'll most likely have to buy from a specialty store online. Sterilite, one of the largest manufacturers of plastic storage containers, claims their products are made from polypropylene and polyethylene, with the clear containers being acid-free. However, unless they've been tested under P.A.T. - the Photographic Activity Test - there's no guarantee these Sterilite containers are beneficial for your items (much like the acid-free cardboard boxes from a dry cleaners for a wedding dress, under most circumstances, they are not in fact 'acid-free.')
The Photographic Activity Test was developed by the Image Permanence Institute to determine the level of susceptibility of a photograph (one of the more sensitive mediums when it comes to the agents of deterioration: light, temperature, humidity, pests, water, pollutants, etc) in an enclosure. The test determines the rate at which a photograph will deteriorate, or if any staining will occur while in the enclosure.
Even if you do use a polypropylene container to store your objects, you'll then have a problem with microclimates, or the creation of a interior climate where the temperature and humidity are different than its exterior surrounding area. Sometimes this can also be harmful. As I mentioned earlier, polypropylene is chemically inert, so it won't off-gas, but that doesn't mean your objects won't. By sticking your item in a container where in can't breath (typical archival paper boxes are breathable, where plastic is not), you're effectively trapping whatever chemicals your object gives off in with the object as it starts to break down, again speeding up deterioration. This is the case with plastic objects, such as Neil Armstrong's space suit worn when he took the first steps on the moon, or cellulose nitrate films from the 1920s-1950s, which spontaneously combust as they off-gas.
It's all about give and take with collections, but that's what I find fascinating. There's also the philosophical question of 'if it's going to deteriorate anyways, why keep it,' but I try not to dwell on that question too much, as it's just too sad.
Still questioning if your plastics are ok for your items? A good rule of thumb is to look for recycle code 5 (or PP) on the bottom of the enclosure. If you can't find a number, don't use it, unless you know for sure that it has come from an archival supplier. Otherwise, ask a museum specialist for their help preserving your items.
For those really interested in the breakdown and storage of plastics, check out this really great article by the Getty Conservation Institute).