O. Sami Saydjari wrote:
When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC
region, they say something like, "Midway altimeter is xx.xx." I have
some questions.

1. So, do they pick some fix for the region and give everyone (those
below FL 180) the altimeter of that fix? Do they break up into
sub-regions and give everyone in that sub-region that same fix altimeter
setting?

You should get the altimeter setting for the airport nearest your
location which has weather reporting.

2. At what point after departing an airport do you change altimeter
setting to the one supplied by ATC?

I update my altimeter setting whenever ATC gives me a new one.

Similarly, on approaching an airport to land, at what point do you
switch over to the altimeter setting of the airport (from that given
by ATC).

If ATC has given me a setting for my destation, I'll just use that.
If not, then I'll set my altimeter whenever I get the ATIS/AWOS.

The reason I ask is that sometimes, the altimeter settings can be quite
different.

What do you consider "quite different"? Unless there's some severe
weather going on, it's unusual to have to change the setting more than
a couple of 1/100's at a time.

Especially if there is VFR traffic around an airport, it
seems that there is some risk if everyone entering the pattern (or
approaching the pattern) does not share the same altimeter setting.

Risk of what? If somebody's 50 feet high or low in the pattern, it
doesn't make much difference. As long as it's VFR, see and avoid is
the rule of the day. The real reason for having sensitive altimeters
is for instrument work.