Hello, I'll take this one.
Glass would be a much better option, how ever you have plastic so thats what we have to work with. Your husband most likley left the containers sitting on the bottom of the pan also. A safer technique might be to utilize a container floating within another container or elevate the bottle off the bottom with a steamer insert. Water is about 212 deg. f when boiling, I don't know what type of plastic your bottles are made of. Certain plastics have thermo propertys that would work in what your trying to accomplish.
First of all you need to identify what type of plastic the bottles are made of.
Some plastics give off toxic fumes when heated/melted/burned.
There are potential hazards in what you just encountered, and the conditions cannot be controlled (e.g. temperature and fumes generated by decomposition).
Your husband would have first noticed a softening (rather than melting) polymeric plastics depends upon what the plastic is.
Two dangerous things happen when plastic burns, depending upon the material.
1. It may catch fire. 2. It may give off toxic fumes. In particular polyvinyl chloride (the stuff water pipes are made with) decomposes to produce HCl and other nasty fumes. Be advised your containers are not made of PVC.
Some plastics (for example Bakelite, the black plastic on pots and pans) will not soften at all it will just char, giving off noxious fumes.
HDPE is partially crystalline. This raises the softening point so high that in the presence of air (oxygen) it burns before it melts.
Polystyrene, when heated to high enough temperature to soften it "unzips" and produces styrene monomer which is toxic and flammable. Polystyrene burns with a very smokey flame and will set off your smoke detector in an instant.
Polymethylmethacrylate (Plexiglas is the trade name) softens at about 100 C but it does not really "flow" very much. It too tends to "unzip" to give off methyl methacrylate monomer.
Then again it could be a polycarbonate a similar material as what plastic sunglasses are made of. It's a very clear and hard plastic and it's melting point is 310 deg. f but could start melting at 240 deg. f over a period of time.
There are several measurable unknown units of data from your recent accident that would have to be answered before anybody could give you an answer.
1) Type of plastic and it's known propertys
2) Highest Temp. reached of plastic
3) Duration / dwell time plastic if it was or was not emitting a gas.
4) Ventilation / air 02 turn over in your space
5) Distance between lungs and gas emitted
6) Ambient temp.
7) Each exposed humans amount intake and strength level of their immune system.
As you see what you're asking cannot be answered without the proper facts.
Count your blessings he did not burn the house down and next time advise him to standby and monitor the short process of sterilization. The fact nobody seems to be coughing or have a raw and damaged throat, I'd say the lungs should be okay. As for the cats, they were probably the safest because they were low to the ground and tend to evade disasters with more distance. I would be interested to know what the cats did because their senses are 6 times greater then humans. Your cats smelled disaster and may have run for the far corner of the house. Some cats are known to wake up family members during incidents like yours.Next we can review what your husband can do to prevent it from happening again.
Make sure all bottles, nipples, and other utensils you use to prepare formula, or to feed your baby, are clean. If the water in your home is chlorinated, you can simply use your dishwasher or wash the utensils in hot tap water with dishwashing detergent and then rinse them in hot tap water. If you have well water or nonchlorinated water, either place the utensils in boiling water for five to ten minutes or use a process called terminal heating.
In terminal heating you clean, but do not sterilize, the bottles in advance. You then fill them with the prepared formula and cap them loosely. Next, the filled bottles are placed in a pan with water reaching about halfway up the bottles, and the water is brought to an observed
gentle boil for about twenty-five minutes.