Virus scan on a macbook air?

Since there are no virus on Macs, AV software is not necessary. They are noting but scams. The only thing they do is use up system resources to cause problems.

Please explain to us why you think you need to do that.

You can, but as of now none is needed. OS X now has its own built-in anti-malware software that is kept updated behind the scenes.

If you are experiencing odd emails or browser pop-ups claiming your Mac has viruses, please ignore them. They are phishing scams:

Identifying fraudulent "phishing" email

Beware of Browser Pop-Up Tech Support, Phishing Scams

Phishing & Other Suspicious Emails