is there an instrument that measures the percent or oxygen in air.?

I have heart trouble and some places I go I can walk for miles and some places I can't walk a block.

Answers:
you may try this
http://www.semedicalsupply.com/pulse_oximeters.htm

There must be, but I haven't heard of one for personal use. Keep close monitering of the weather channel; they should tell you.
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