Is ground level ozone dangerous to human health when other pollutants are not present, for example, at high altitudes?
Yes high ozone levels are bad for human health and plant development.  

Integral to your question is the natural high ozone concentrations at altitudes where you are near the tropopause.  The stratosphere has a lot of ozone and it gets entrained to the air around high elevation mountains quite often.  

You could read what the US EPA has to say about ground level ozone: http://www.epa.gov/airquality/ozonepollution/ which is relevant in terms of the health effects and the ecyosystem effects links.

It says: "Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground level ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue."

The National Ambient Air Quality Standards for ozone is 75 ppbV for an 8-hr average.  This means that if a county has 4 days per year with exceedances of the 8-hr avg threshold (over a 3-yr average), the air quality does not meet the Federal standard.