A recent Associated Press article tells us that Microsoft's backup power plans at their Quincy, MA datacenter are being challenged by a group concerned about diesel emissions. Microsoft is looking at adding 13 diesel generators as standby power sources, and the group challenging the plan wants the state Ecology department to require that the generators have filters, presumably on their emissions.
In the US the EPA has implemented a series of standards for diesel engines that directly address pollutants, with the most stringent standard known as Tier 4i (interim) due to take effect on January 1, 2011. These standards directly address the issue raised by the group in Massachusetts, but for a very specific reason, have no impact: stationary emergency generators are exempt from the standard.
There's a good reason for this. First off, emergency generators, over all, are responsible for only a minuscule piece of the potential pollutants put in the air by diesel generators. The regulations are primarily targeted at those engines which would do the most potential environmental damage. Second, by their very nature, the purpose of emergency generators is to provide power in an emergency, which means that, ideally, they are as flexible as possible. Utilization of the current state of the art filtration systems requires the use of ULSD (Ultra Low Sulfur Diesel) to prevent the filters from being clogged. In the event of an extended emergency, the availability of ULSD may be quite limited, and the ability to use standard diesel fuels a major advantage.
This isn't to say that diesel emergency generators should have a broad regulatory exemption; they are actually still required to meet Tier 2 or Tier 3 levels (depending upon their power output). These earlier regulatory standards were the driving force behind the current generations of much more efficient and lower polluting diesel engines. Nor does it mean that higher standards will never be applied to the unique situation of the emergency power backup generator. But it does mean that pursuing ad hoc requirements for diesel exhaust emissions filtering will add cost and limit flexibility.