Skiing and climbing in the Rockies may put you at risk, and some people will be affected in the towns and cities. For example the town of Leadville, Colorado is above 3,000m. The major mountains of North America are in the northern latitudes, where air pressure is lower for the same altitude than at the equator.
In the Andes it is possible to fly or even drive into places such as Cusco (3,326m), or La Paz (3,600m) without any acclimatisation en route. You should rest for the first few hours then take things easy for a few days before trying one of the high treks such as the Inca trail.
FIFA have banned international football matches in La Paz in Bolivia since the home team have a major advantage due to their natural acclimatisation to the lack of oxygen.
A businessman hoping to clinch a multimillion dollar contract flew to La Paz (3,600m) in Bolivia the day before an important meeting. His company had decided to save on the expense of sending him a few days early to enable him to acclimatise. He felt so ill when making his presentation that he lost the contract. The next time he will make sure he gets the chance to acclimatise.