Apparently there are various grades of Mylar pouches (one of the companies I work with switched from bags from some Asian seller to a marginally more expensive North-American company because the first one did not have any explicit certification as food grade, even if they don't pack food in them).
The difference apparently is in the composition of the inner layer when the pouches are the usual laminated ones that can be heat sealed at home. (apparently some are laminated with LLDPE others with VDC and others with some non-described materials. Since coated bags are normally used at home, I didn't bother too much with handling temperatures for the uncoated Mylar)
This is from a Mylar booklet by DuPont Teijin Films:
General Properties To impart heat sealability and improve barrier
properties, certain types of Mylar are coated on one or both sides
with copolymers of which vinylidene chloride is the predominant
monomer. Vinylidene chloride copolymers are highly resistant to
oxidation and biodegradation, which make them extremely durable under
ambient conditions (Ref. 16). When heated above 120°C (248°F), the
copolymers may decompose and evolve hydrogen chloride only up to 190°C
(375°F) (Ref. 17). Other films are coated with acrylic copolymers or
aluminum metallization (with or without vinylidine chloride copolymer
Toxicity Vinylidene chloride copolymers of the type used on coated
Mylar have been used as food packaging components for many years.
Cellophane coated with these same copolymers has been fed to rats and
dogs for two years at a dietary level of 5% without any ill effects
attributable to the coating.
Thermal Stability A volatile product of vinylidene chloride
copolymer thermal degradation is hydrogen chloride (Refs. 17–19).
Hydrogen chloride is an irritant for which OSHA has set
a maximum permissible exposure of 5 ppm (OSHA Title 29, Code of
Federal Regulations, 1910.1000). One of the functions of the
vinylidene chloride coating is as a heat sealant when the coated film
is used as an overwrapping material. Overheating may lead to
generation of hydrogen chloride. (Refer to the “Handling Practices”
And further down in the booklet under FDA Status:
All Mylar made and sold for food packaging complies with
FDA Regulation 21CFR177.1630— Polyethylene Phthalate Polymers. This
regulation describes polyester films that may be used safely in
contact with all types of food. Polymer coated films are restricted to
use below 120°C (250°F), but uncoated PET films are specifically
cleared for use in oven cooking or baking at temperatures above 120°C
(250°F). (DuPont recommends a maximum oven temperature of 204°C
[400°F].) The US Dep't of Agriculture has accepted Mylar® for
packaging meat and poultry products prepared under their inspection.
One company (sorbentsystems) includes this line in some product description (but not on every mylar product):
These bags are designed to withstand high temperatures (up to 250° F
for 55 minutes) for hot fill. Food Grade.
I would presume that any bag specifically made to be hotfilled would be able to resist having hot water poured in and that just filling with hot water would also not be able to maintain an elevated temperature for too long to worry about it. But, as these things go, not all the pouches are equal to eachother and there could be a lot of variation from brand to brand, or different products of the same brand.
As it seems there is only a general answer and it would be wise to make a bit of research, or contact the manufacturer directly, to know the specific materials of the bags one is using and purchase from a known and reputable manufacturer that publish all the needed materials specification and certifications when in doubt.
PS: when the above quoted text says to see the handling practices section its referring to a paragraph concerning fumes coming from outer coating buildup on the heated elements of sealing machines in commercial operations, conductivity of the film and scrap disposal.