Types of macbook power adapters and general rule of thumb
There are 3 kinds of power adapters for macbooks in the market: 45W, 60W and 85W. The general "rule of thumb" is that it's ok to use a charger with higher wattage than the one that came with your device, but you should avoid using a lower wattage one.
Using 85W charger for Macbook Air
By default Macbook Air's original charger uses 45 watt and Macbook Pro's 85 watt. Can Macbook Pro power adapter (85W) be used to charge Macbook Air? The answer is YES - you can charge Macbook Air with Macbook Pro's charger without any concerns - 85W is just a possible maximum rate, so Macbook Pro's power adopter will only supply what is requested. It will never just automatically run at 100% of it's capability. If you will be charging macbook air (that is supplied with a 45W adapter) with an 85w adapter instead, it will still only draw what it needs - so, it's perfectly fine.
Official reply on using higher power adopters from Apple
There are a lot of experiments in the Internet - they say that it's ok and battery life doesn't get reduced if charging with power adopter of higher wattage. The official reply from Apple: "You can use a higher wattage power adapter, but you cannot use one with less wattage without potential operating issues." (https://support.apple.com/en-us/HT203207).