There isn't a maximum humidity level that someone can apply paint. The reasons are that different products react to humidity differently and high levels (70% or more) drastically slows drying and curing.

Basic recommendations for painting during high humidity;

1) The surface can't be physically wet.

2) High humidity levels will slow down drying. What was "dry to the touch in 2 hours" can become 4,5 or even 6 hours if the levels are high enough. You need to keep an eye on the sky looking for bad weather. Even a light rain can and will ruin a paint job if the paint hasn't fully dried.

3) Acrylic primers and paints can deal with humidity better than oil based products.

4) The perfect levels are 40-50% humidity or lower. If you can wait for this type of weather it will be worth it.

Humidity is one part of the equation, temperature also plays an important part. Higher temperatures will help the drying process even at high humidity levels. Low temps could cause dew to form, evening and late hours, and this water could cause problems that you will have to deal with. If the humidity levels are high and the temperature is low, 60 degrees, then the paint will take even longer to dry.

I just looked up this question because I'm painting the wood exterior of my house in mid October in southern British Columbia using a special paint developed for cold weather application (CIL Cilux permaflex – can use at 34F). I had no problem painting in 36-40F and it said nothing about humidity, just that it may take up to 24 hours to dry in low temperatures. I painted yesterday afternoon in 60F under nice sunny skies. We had a light rain overnight and this morning but this siding is completely under cover of the front porch. I went out to paint apply a second coat 11am this morning when the rain had stopped. However, the humidity was 95 percent and temperature 50F. I was surprised to find it had a wet greasy feeling to it as if dew had collected on it. I was able to wipe it down with a clean cloth and just a little of the color came off onto the cloth. I'm just hoping it dries out enough this afternoon to apply a second coat before winter really sets in. Humidity is very important for drying. Something the can may not mention.

I've been painting outside columns with a satin semi-gloss latex and having drying problems. Close to the water in Panama City Beach Florida, the humidity has been around 90%. The temperature is around 75 and the paint after 2 days still hasn't dried. In fact much has rolled off onto the driveway. It seems a light fog has been rolling in every afternoon which coats everything in dew. I'm hoping 70% humidity and sunny today will be enough to finish the job though its 5 degrees cooler.