TLDR: It looks like the bigger risk is spilling the boiling water onto yourself.
Here is what Ziplok has to say about their products,
That’s good news all around.Ziploc® brand containers are made from polypropylene and can be recycled in a limited (but growing number) of communities.
...
SC Johnson’s Ziploc® brand Bags and Containers are BPA free. Our products are extensively evaluated for toxicity and safety and comply with applicable quality and safety regulations.
...
A few years back, concerns were raised about the alleged dangers of using plastics in microwaves. The presumption argued that a combination of fat, high heat and plastic releases dioxin into food and ultimately into the cells of the body, which would then increase the risk of producing cancerous cells. After we researched these claims, it became clear that the information was misleading and unnecessarily alarming.
Source
According to National Geographic the is used in,
- deli soups, syrup, yogurt and
margarine containers
- disposable diapers
- outdoor carpet
- house wrap
- clouded plastic containers,e.g. baby bottles, straws
and there are
No known health issues
Source
From a SAFETY DATA SHEET Polypropylene (PP)
Ingestion:
No known significant effects or hazards
...
Melting Point | 150-170 °C (DSC)
Decomposition temperature | >300°C (>572°F)
...
Chemical stability:
This product is a stable thermoplastic with no chemical reactivity under normal handling and
storage conditions.
Possibility of hazardous reactions:
Under normal conditions of storage and use, hazardous reactions will not occur.
Conditions to avoid:
Strong oxidation agents, avoid temperatures above 300 degree C (570 F).
This article says that polypropylene is considered less risky for leaching compared to other types of plastic..
There appears to be even less concern – which has translated into fewer studies – regarding leaching of toxins from polypropylene (recycle code 5), the other plastic dinnerware workhorse. Polypropylene bowls and plates are also considered safe to use in the microwave.
What about BPA and phthalates?
What we do know though is that neither polypropylene nor melamine contain two of the toxins that have raised concern in recent years: bisphenol A (BPA) and phthalates.
Other notes I found,
All of the plastic food containers that we examined in our kitchen were branded with the number 5, which means they’re made of PP, or polypropylene. That’s a cloudy plastic generally considered to be food safe.
Source
Not so, says vom Saal, who notes that there are plenty of other materials, such as polyethylene and polypropylene plastics, that would be fine substitutes in at least some applications. "There are a whole variety of different kinds of plastic materials and glass," he says. "They are all more stable than polycarbonate."
Source
I also found a study where the microwaved water in plastic containers and tested for leakage.
The Effect of Microwave Radiation on Polypropylene Used in Food
Containers
The question of whether or not any potentially hazardous organic
compounds were being introduced into food by the plastic itself was also addressed. Although there were not any
specific answers given to these questions, the quality of the plastics commercially available were evaluated. The
results proved that the plastics used in everyday food containers are of good quality and do not leach enough
hazardous material to be in anyway harmful.
...
The results indicated support of the original
hypothesis. No evidence was found of any harmful
chemicals leaching into food from the plastic dishware.
