First, get your possible gas leak checked out right away. Call the gas utility company that services your house and ask them to come and check for a leak. Most gas companies will do this for free and quickly, because it is not good for business when their product kills people. If you suspect any gas leak at all, you should get it checked.
Second, whether your security bars are actually secure depends on how they are installed. Are they fastened securely? If you have a sash window like the one in the picture, usually both sashes can be moved up and down, but it's possible that in your installation the upper sash was not designed to move or has been fastened in place.
Regarding closing windows on upper floors for safety: usually only ground-floor windows need to be secured, as thieves tend to look for the easiest opportunity, and the challenge of climbing into an upper story window probably isn't worth it. Take a walk around your house, think about the top few ways you might get in if you lost your keys, and secure those things first. And let yourself enjoy a little fresh air at night. :)
We have used DE effectively to kill bedbugs, so I'll take a shot
how should it be applied?
We applied it around the borders of all the floors, sprayed it into the walls everywhere we could (mainly into electrical boxes - remove the covers first), and into the underside of plush furniture. In the basement we put it all over the ceiling and walls as well as squirting it into every gap we could find.
is it effective against bed bugs? How long does it take to kill them?
Yes. They will avoid it if possible, so its tough to say how long it takes to kill them, but when we applied it to a heavily infested house and the improvement was immediate.
where should it be put? Everywhere, or only in specific locations?
Anywhere bedbugs can get.
how much should be applied?
A dusting around the perimeter of each room, a generous squirt of dust into openings and several squirts into plush furniture is what we did.
how often does it have to be reapplied, and does it have to be vacuumed up?
We only applied it once and that did the trick. If you vacuum it up it can't help, so no don't vacuum it up.
after it's been applied, will a fan or wind disturb it and blow it into the air?
Depends on the orientation and strength of the fan. A ceiling fan? No. A box fan pointing at a line of DE? Probably.
is it safe to apply it on a bed?
You should seal your mattresses up with bedbug preventing covers, and squirt the DE inside that. DE is harmless to you, but you want to trap the bugs in the mattress. Leave the mattresses sealed for at least 2 weeks or longer to ensure the bedbugs get exposed. They can hide and live for a long time in a mattress but a solid month should deal with the birth cycle.
is it effective when wet? when the air is humid?
No and not very.
Best Answer
Standing stagnant water can harbor legionnaire's disease, be a mosquito breeding ground, etc. And as @mike says, regional hazards vary, check with your local health authority, what works here in Oregon might not work for example, in Florida, etc.