US chlor-alkali facilities operated at 86% capacity in May, according to data released Friday by the Chlorine Institute, the highest effective operating rate since the 87% seen in May 2013.

May 2014's operating rate represented a 5-percentage point spike from the 81% seen in both April and March.

US chlor-alkali producers made 852,131 st of liquid chlorine, 1,044,322 st of chlorine gas and 1,099,203 st of liquid sodium hydroxide in May, the trade association said. The monthly production corresponds to daily rates of 27,488 st/d of liquid chlorine, 33,688 st/d of chlorine gas and 35,458 st/d of liquid sodium hydroxide, it said. The total volume of sodium hydroxide was the highest since January 2011's 35,658 st/d, when production rates were at 92%.

In May 2013, daily production rates were 26,012 st/d of liquid chlorine, 32,275 st/d of chlorine gas and 33,249 st/d of sodium hydroxide.

Companies reporting monthly data to the Chlorine Institute account for 93% of US capacity.

Market sources said that strong water purification and downstream vinyls demand -- chlorine is one of the main feedstocks for PVC -- were the driving factors behind the increase in operating levels.