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"  Public health officials are routinely concerned about cross connections and back-flow connections\u2014the links through which contaminating materials can enter a potable water supply\u2014in plumbing systems. Public School 20, in Flushing, Queens, confirmed the worst fears of these officials when, earlier this week, its drinking water was contaminated with an air-conditioning chemical\u2014most likely propylene glycol\u2014that sent home 74 children who were vomiting or had stomachaches.  First of all, let&amp;#039;s look at cross connections\u2014a plumbing problem that can get seriously complex. To begin with, think of how confusing the plumbing is just around your boiler. Now multiply that exponentially for a large building, which can have miles of piping. Given time, the system becomes ever more complex as components are added and other parts of the system are changed. In the worst case, a redesign of a building&amp;#039;s plumbing system will lack proper documentation and a mechanical contractor or a plumber will have no drawings to use as a reference. In a scenario like that, a cross connection can become likely. Some would even argue inevitable.  In other cases, it can be a simple mistake or it can be due to pipes that are incorrectly identified. In still other instances, components that are designed to prevent back flow or cross contamination fail or are not correctly installed. The particulars on the cross connection for Public School 20 are being investigated. &amp;quot;They&amp;#039;re still looking into it,&amp;quot; says Department of Education spokeswoman Marge Feinberg of the incident, which may have occurred when a contractor was called for repair work on a recently installed central air-conditioning system, located on the rooftop of the school&amp;#039;s new wing. The contractor&amp;#039;s company, Bayside Refrigeration, was unavailable for comment.  With an investigation still pending, we went to the experts\u2014professional plumbers\u2014to see how they think this accident could have occurred.  Mechanical confusion, breakage or misintallation aside, how do fluids in a plumbing system flow in the wrong direction? Answer: A pressure differential that works opposite the direction that it would normally. For example, when there&amp;#039;s a massive leak in a municipal water system, the pressure can be higher where the water is delivered (inside houses and buildings) than it is under the streets where the water mains are. If back-flow preventers haven&amp;#039;t been installed or if they fail, this will cause the water to flow out of houses and buildings and into the leaking water system. &amp;quot;Simply put, someone or something inadvertently caused more pressure on one side, where the chemicals were, and it got flushed back into the domestic side,&amp;quot; says Pat Porzio, a master plumber, mechanical engineer and HVAC manager for Russo Brothers &amp;amp;amp; Co. in Cedar Grove, N.J  Porzio adds that if the pollutant was, indeed, propylene glycol, then it could only be used with two kinds of air-conditioning systems. &amp;quot;Glycol would either be in a heating coil,&amp;quot; he says, &amp;quot;or in a cooling coil, if they&amp;#039;re running chilled water through it, and they&amp;#039;re running it up to a roof, and there&amp;#039;s a chiller system. Because it&amp;#039;s outside, they want to put some glycol in it to keep it from freezing.&amp;quot; Most likely, since the air-conditioning system at P.S. 20 is on a roof, the unit involved has a cooling coil.  Keith Mandel, who runs Keith&amp;#039;s Plumbing &amp;amp;amp; Heating in Long Island, N.Y., believes that the mix-up may have occurred during installation. That is, the contractor could have confused the domestic water pipe with the air-conditioning pipe. &amp;quot;My feeling is that the air-conditioning mechanic might have mistaken one of the copper lines for the air-conditioning line and not the plumbing line,&amp;quot; he says, adding, &amp;quot;This doesn&amp;#039;t happen. I don&amp;#039;t think I&amp;#039;ve ever heard of this happening. There are so many cross-connection controls taken in New York and Long Island by the Department of Health for the back-flow devices.&amp;quot;  However, because the units are new, says Mandel, the back flow may have occurred when the contractor turned the system on. &amp;quot;The contractors might have tapped into a existing water line,&amp;quot; he says. &amp;quot;And then when they charged the system, it went into the water lines, rather than a closed system just for the air conditioning.&amp;quot;   Regardless of the reason why P.S. 20&amp;#039;s water was contaminated, 74 students were rushed to area hospitals all because of one simple plumbing mistake. While no injuries were life-threatening, this incident still provides an important, if not essential, lesson to plumbers working with cross connections. For now, jugs of fresh water will be delivered to the school\u2014and the water fountains will remain shut off until further notice. "^^<http://www.w3.org/2001/XMLSchema#string>
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