Altitudes of 1000 feet or 300 meters or more requires adjusting your standard diving rules to compensate for the relative difference in the atmospheric pressure.
Diver training programs assume that you are diving at or near 1 ATM ( at or near sea-level). When your diving altitude is much higher you must compensate for the reduction in atmospheric pressure. For example, at say 1000 ft you are at .96 ATM and at 10,000 ft elevation you are at .69 ATM.
One of the key factors to consider is your surface interval.
When you plan your dive, you need to add two pressure groups for every 1,000 feet difference in elevation. San Francisco is near sea level while Lake Tahoe is slightly over 6,000 feet above sea level. You can adjust your pressure group by taking into consideration the time between your arrival and the start of your dive. It will take about 6 hours for your body to naturally adjust to the point where you do not need to add the pressure groups. If you were going to a location with an 8,000 feet or more difference you must wait 6 hours before diving. USN tables suggest a 12-hour delay.
You must also consider your Theoretical Depth when at high altitudes.
A 100 foot/30 meter dive starting at 9,000 feet/ 2,700 meters. Your theoretical depth is now 140 feet/ 42 meters which is beyond the depth listed as the maximum for recreational scuba diving. If you were diving from a 10,000 foot / 3000-meter elevation you would exceed maximum depth at 90 feet/ 30 meters. If you do not adjust your dive time to include the theoretic depths you could be increasing your risk of DCS.
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