what's the cabin humidity to be maintained...
 

1)Is it true that the british authorities require the cabin humidity to be maintained at least 30% RH? how is it done i heard even dreamliners can maintain only around 15%...how do airlines comply then if that is the case? also what's the scenerio in other places?

There is no minimum RH specified.
It can get as low as 5% in premium cabin.

See https://www.flightglobal.com/news/ar...le-cab-369425/

The requirements of an air-conditioning system as laid down in BCAR’s are described below.
Provision of fresh air
Fresh air must be provided at a rate of 1 lb per seat per minute in normal circumstances, or at
not less than 0.5 lb following a failure of any part of the duplicated air-conditioning system
(No JAR Figures quoted except for crew which is “not less than 10 cubic ft per minute per crew
member”)
Temperature
Cabin air temperature should be maintained within the range 65°F to 75°F, (18°C to 24°C).
Relative humidity
The relative humidity of the cabin air must be maintained at approximately 30% (at 40,000 ft the
relative humidity is only 1 to 2%).

THIS EXCERPT STATES OTHERWISE AND IS WHY I ASK...IT'D BE GREAT IF SOMEONE FLYING THE BRITISH SKIES COULD CLEAR THIS UP...!!

Good question.
I would be very interested to know what range we have onboard our 737's.

Please provide a link to your BCAR.

See House of Lords Science paper in 2000 House of Lords - Science and Technology - Fifth Report

See: 5.29
"There are no specific regulatory limits for cabin relative humidity."

"5.33 The definitive experimental work on this topic was carried out at the then RAF Institute of Aviation Medicine (now DERA) at Farnborough, Hampshire. This showed clearly that exposure to 5% RH for 24 hours did not lead to changes in overall water balance amounting to central dehydration (p 72). Professor Nicholson also showed that the maximum increase in water loss for a person spending 8 hours at 0% RH was about 0.1 litre, well below even the thirst sensation level[74]. He, Dr Sowood (p 72), and Professor Denison (p 94, Q 214) were of the firm view that any extra water loss due to the dry cabin environment is of no significance to health, and that central dehydration of passengers in low humidity aircraft cabins is a myth. As Dr Giangrande noted (p 234), the assertion in Q 108 that breathing dry cabin air means that passengers are not replenishing their blood plasma is nonsense."

tnx that was helpful...the excerpt is actually from a textbook so not sure if that requirement actually exists...hence i asked.