For welding, Speedglas Adflo is the bees knees, but pricey at upwards of 1800 clams. 
3M do the versaflo visors which can also either run off a battery powered unit that provides positive pressure within the mask, or an air filter with in line oil filters and driers. Big dollars too. 
Why do you require 'positive' air helmet? What exact work do you plan to do?
I'd recommend that you consult the SDS / MSDS for the PPE requirements before dropping the coin. 
P1 masks range from cents for the disposables, up to 5-600 for the full face glass 'breaking bad' style units like the 3m 6900 optioned to the hilt with the full retard filters. 
Basics liek the P1 filters are good for dust and particles. P2 filters for thermally generated particles (think grinding / welding) and then you get into the 'alphabet' A1B1 E and K organic vapour filters for high volatility solvents etc... 
Best advice is know what you work with, and use a selection guide from any of the manufacturers* along with the MSDS to work out how crazy you should go. Yeas you need to look after your health, but positive air is proably overkill for the average person's requirements. 
*Manufacturers include
3M
MSA
Speedglas
Protector
Honeywell
Also check out the paftec cleanspace 2.
				
			 
			
		 
			
				
			
			
				-Mitch
'El Burro' 2012 Defender 90.
			
			
		 
	
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