I recently purchased a condominium on the 35th floor (in Tokyo), but
was informed that prolonged high altitudes for an infant was not good.
 Is this true?  If so, are there ways to alleviate this problem?

Just a free comment.
"High altitude" is considerably higher than the 35th floor in Tokyo,
which is probably 300-400 ft above sea level, or +/- 150 meters.

Here is a definition of high altitude:
"High altitude are regions on the Earth's surface (or in its
atmosphere) that are high above mean sea level. The composition and
temperature of the atmosphere at high altitude is substantially
different than at sea level. These differences can affect living
organisms, including humans. High altitude is sometimes defined to
begin at 1500 m above sea level."
en.wikipedia.org/wiki/High_altitude

I hope this calms your fears.

Let's assume the floor-to-floor height is 12 feet. The 35th floor is
then 420 feet above the grade of the building. Tokyo is essentially at
sea level, but let's add another 30 feet for the elevation of the
first floor. 450 feet hardly qualifies as "high altitude". Millions of
babies grow up normally in places like Mexico City (7400 feet),
Denver, Colorado (5300 feet), or even La Paz, Bolivia (12,000) feet.
If you live at Sea Level and travel to a place like La Paz, you will
experience the low air pressure and may get quite sick from it, but
450 feet is no problem at all.

The only problem I could imagine is the change of pressure when you
ride the elevator. I've never lived or worked 35 floors up and I
suppose you might feel your ears pop if your head is congested, but
the pressure differential is so small it is not going to do any
damage.

I wonder if it's got more to do with the air pollution and smog being
more locally concentrated at a higher level than at ground level. I'd
be more concerned with safety issues -- for example, if I had an
infant, I'd be permanently gluing shut every window and making sure
that the fire brigade could get to me if necessary!

This question would only be pertinent if you intend to raise your baby
in the Himalayan mountains. ;-) That heigher you're referring to will
not pose any problem at all!

Enjoy parenthood!

At that height the daily change in barometric pressure is going to be
greater than the pressure differential between sea level and 500 feet.
 Don't worry about it.

Mexico City is at 7200 feet, and there are literally millions of kids
running all over the place. I am at 5700 feet in rural Mex. and there
are zillions of healthy kids.

Now, there is a city, can't remember, maybe Quito or Lima, which is
near 20,000 feet and that might be tough on a kid without the Indian
ancestry which involves very large lungs and so forth.

My view is if the mother can survive to give birth at that altitude,
the baby will probably be fine.