Sewage sensors to detect bomb making factories Mr Ostmark said Emphasis, which stands for Explosive Material Production Hidden Agile Search and Intelligence System, has already been trialled in Sweden. He added: "We are seeing intelligence that could help the police." The sensors can sit in the flow of waste water or sewage tunnels and are made of "ion selective electrodes." These change voltage when chemicals used in bombs pass through them, and this can be tracked to find out where the chemicals are coming from. The system, which is being led by the Swedish Defence Research Agency, will also use static sensors to measure vapours in the air, that emerge during the bomb making process.