Many of us have an overflowing kitchen cupboard of plastic containers to store our leftovers.

But as awareness grows over the health and environmental pitfalls of plastic, some consumers may be wondering: is it time to ditch that stash of old deli containers?

Only 9% of all the plastic waste ever created has been recycled. From its contributions to global heating and pollution, to the chemicals and microplastics that migrate into our bodies, the food chain and the environment, the true cost of this cheap material is becoming more apparent.

There are thousands of compounds found in plastic products across the food chain, and relatively little is known about most of them. But what we do know of some chemicals contained in plastic is concerning.

Phthalates, for example, which are used to make plastic more flexible and are found in food packaging and plastic wrap, have been found by the Centers for Disease Control and Prevention (CDC) in measurable levels across the US population (including in the body of Guardian journalist Emily Holden). They have been linked to reproductive dysfunction in animal studies and some researchers have suggested links to decreased fertility, neurodevelopmental issues and asthma in humans.

BPA, another chemical widely added to food plastics, has been subject to increasing regulations after studies linked the chemical to neonatal and infant brain and reproductive harm. But BPS and BPF, two common replacements used in products marketed as “BPA-free”, may have similar effects to their predecessor: studies out of both the University of Texas and Washington State University found that even at a dose of one part per trillion, BPS could disrupt cell functioning. A 2019 study from New York University linked childhood obesity with BPS and BPF.

There are many other chemicals added to plastic during production, and researchers concede that many gaps remain in our understanding of how they affect health and development. But research that is adding to concerns about the “miracle material” is growing.

What’s in those takeout containers?

Food containers are just one link in a massive chain of plastic products that touch things we eat, from coated conveyer belts in food production lines to disposable clamshells for delicate berries, clear carrot bags and milk jugs.

Researchers say it is difficult to answer which plastic containers are safe without greater transparency about what chemicals make up everyday plastic materials.

In 2019, the Food Packaging Forum (FPF), a Switzerland-based not-for-profit focused on the science behind food packaging, compiled a database of more than 900 chemicals “likely” associated with plastic food packaging production worldwide and another 3,400 “possibly” used. Of those 4,300 chemicals, 60% did not have any available hazard data, researchers found.

“[The] ‘known knowns’ are the ones I’m going to gravitate to in terms of concern,” said Dr Leo Trasande, director of the Center for the Investigation of Environmental Hazards at New York University’s Grossman School of Medicine, referring to well-known plastic additives such as BPA and phthalates. “The reality is there are many ‘unknown unknowns’ … that may be as problematic.”

In 1988 the plastics industry came up with standardized identification codes for the seven most common types of plastic resin in circulation. Those little numbers found on the bottom of soda bottles and yogurt tubs clue you in to what type of plastic you’re eating or drinking out of. Most food containers – both takeout containers and kinds meant for reuse – are made of low-density polyethylene (4) or polypropylene (5).

Researchers aren’t exactly sure how much chemical exposure occurs from food packaging and storage containers, but they know plastic isn’t a completely stable material. Trasande said that when exposed to heat – for example, in the microwave and dishwasher – polyethylene and polypropylene can break down, leaching unknown chemicals into food and drink. Oily foods are also thought to attract some plastic chemicals.

The complex chemistry needed to make plastics makes it hard to know exactly what other chemicals are found in plastic food containers, said Jane Muncke, managing director and chief scientific officer at the FPF. (The FPF, one of the few to study food packing exclusively, receives much of its funding from the glass packaging industry, though Muncke said its research priorities are set independently of funders.)

Free radicals and reaction by-products are formed during plastic production so that the chemical ingredients you started with might not be the actual composition of the final product. There are also impurities and so-called non-intentionally added substances (Nias) in the original source materials that accumulate alongside known chemical ingredients.

Not even manufacturers, Muncke said, “know exactly what the chemical composition is of the materials of their product down to the last little molecule”.

Additionally, few studies exist on end-product plastic chemicals, rather than individual source chemicals. In a paper published last year in the Environmental Science & Technology journal, German and Norwegian researchers used samples of real-world plastic items such as shampoo bottles, yogurt cups and refillable water bottles to test their effect on cells in a laboratory. They found compounds in consumer plastics that are toxic in vitro, but are largely unidentified.

The Plastics Industry Association (Plastics), a trade group, disputed researchers’ claims of questionable safety.

“All plastics used in food packaging go through rigorous testing. By the time any type of plastic food packaging even makes it to the US Food and Drug Administration (FDA), it’s already been tested several times by chemists and toxicologists. Plastic food and beverage containers may be used safely in the freezer, microwave, dishwasher or a combination of all three when these uses are labeled on the package,” a spokesperson said in an emailed statement.

Who is testing those food containers?

The FDA has oversight of any “food contact substance” (FCS), a category that includes reusable food storage and takeaway containers, as well as food manufacturing surfaces and original packaging.

But some experts question the rigor behind the FDA’s standards, in part because the agency relies on manufacturers to determine the safety of any chemicals used across the food system.

The FDA reviews toxicological data from manufacturers on any new FCS. But the agency doesn’t regulate how manufacturers perform their testing; instead the agency provides “recommendations”. And even those recommendations don’t align with the latest science on chemical toxicology testing, said Maricel Maffini, an independent consultant and expert on food additives.

For example, manufacturers don’t actually have to provide toxicology data for substances they use if they estimate human exposure falls below 0.5 parts per billion (ppb). Yet scientists believe some endocrine disrupting chemicals can have effects at even lower amounts, particularly in vulnerable populations like infants and pregnant women.

The approach also doesn’t account for cumulative exposure. “If you think about all the chemicals that we already know are endocrine disruptors that are in the diet, I’m sure that we may be exposed to more than – cumulatively, collectively – we are exposed to more than 0.5 parts per billion,” Maffini said.

What about claims that plastic containers are microwave- and dishwasher-“safe”?

Under FDA law, it’s also up to manufacturers to set the “conditions of use” of any product, including whether it is “microwave-safe” or “dishwasher-safe”, or even reusable. “It would be up to the manufacturer to ensure that their product is properly labelled for safety under its intended use,” an FDA spokesperson said via email.

Even for containers labeled “safe” for the microwave and dishwasher, experts caution that heat and plastic don’t mix well. Since heat helps to break the chemical bonds in plastic, it’s possible that exposure to high temperatures increases the rate of chemical migration from the container into the food. Scientists have documented higher rates of chemical migration even in water bottles left in the sun.

FDA’s deference to manufacturers on “conditions of use” also means that deli and restaurant takeout containers, which many people reuse and wash in the dishwasher, might not be built for that type of wear and tear. Without more testing and transparency from the FDA and manufacturers, experts can’t say whether they’re leaching chemicals at a higher rate than reusable containers.

There is sufficient evidence that exposure to these chemicals is problematic Dr Leo Trasande

Manufacturers can also rely on ingredients that were approved in some cases decades ago to make their products, even if the science has since evolved to raise new safety questions.

“Once a chemical or mixture or polymer is approved, it is there forever. There is no reassessment or look back to whether what was considered safe in 1962 is still safe now based on advances in scientific knowledge or exposure to that particular compound in the population,” Maffini said.

Trasande conceded “there are clearly still gaps in our knowledge” about large a role food packaging plays in our everyday exposure to thousands of chemicals. Still, “there is sufficient evidence that exposure to these chemicals is problematic”.

He co-authored a 2018 policy statement for the American Academy of Pediatrics focused on the unknown risks that food additives and contact chemicals pose to infants and young children, advocating for more studies and updated regulations.

“None of us are arguing that each and every one of these chemicals are by themselves toxic. We’re not trying to take ourselves back to the 1700s,” he said. “There needs to be new science, and there needs to be requirements for disclosing what chemicals are used.”

How to keep plastic out your food

Given all of these unknowns, along with the catastrophic environmental impact of our addiction to plastic, here are some tips for lowering your plastic dependence and keeping it out of your food:

Switch to glass or metal containers when possible.

Avoid heat, including the microwave and dishwasher, especially when it comes to takeout containers and other forms of plastic not meant for reuse.

Plastic doesn’t last forever (even if some of the chemicals it contains might) – avoid scratched and discolored plastic and pay attention to “expiration dates” on products such as SodaStream bottles.

Don’t store fatty or oily foods in plastic – many chemicals used in plastic are fat soluble and are more likely to leach into fatty food.

Cut down on plastic water bottles, which contribute to the widespread ingestion of microplastics.

Consider replacing your plastic wrap with a reusable option, like beeswax wrap.