The cold fresh air will mix with the warmer room air before touching people. So, you and your customers won't feel cold.
Small or thin openings work well in winter. Cracking a window is better than nothing.
If you only have larger openings, like your front door, use these for short periods of time to replace stale air with fresh air.
Ensure you have a clear plan for when you will replace the air, and who is responsible
Consider using a CO2 monitor to help you estimate your ventiltion levels. This will allow you to adjust your ventilation to bring in the right amount of outdoor air, while minimising the impact of your heating bills. Use the short term options above if possible and work towards longer term changes.
More information on CO2 monitors can be found here.
Where noise is an issue preventing openings consider purchasing an Air Cleaner. These just plug into the wall.
Air cleaners that inlude a fan and a HEPA filter are receommended.
More information on air cleaners can be found here.
Wall mounted fans are easier to retrofit.
Heat recovery systems are available and help to recommended to reduce energy bills.
More information on mechanical ventilation can be found here.