Authorities are continuing to investigate a strong, chemical-like odor that made its way around San Diego County.On Wednesday afternoon, the Air Pollution Control District took two air samples each from Del Mar and Carmel Valley. Those locations were chosen because of the volume of calls coming from those areas and so that the air canisters could be taken quickly from the source to the lab.Each canister was tested multiple times by two separate machines -- one that checks for an ingredient in fuel and another that checks for other toxic compounds.Initially, APCD officials said the final result for the samples put into the machine that checks for fuel ingredients identified slightly elevated levels of six of 56 ingredients found in gasoline.On Thursday afternoon, APCD monitoring chief Mahmood Hossain told 10News the results of the initial testing showed the air was clean when it was sampled. Traces of ingredients found in gasoline were in line with what's usually found in air samples in any urban area. "When we compared the results to similar compounds that we normally detect at El Cajon or Alpine or Camp Pendleton, we do not see any unusual levels," Hossain added. "Whatever it was, it was short-lived." However, a chemist at the APCD told 10News that they may have missed the plume because although reports started coming in about the odor at around 2 p.m., samples were not taken until 5 p.m. from Carmel Valley and at 5:30 p.m. from Del Mar."Maybe we may have missed the plume by the time we got to that sampling location and started collecting the sample," Hossain explained. "The plume may have escaped us." Hossain noted that San Diego usually has an air flow of about 5 or 6 miles per hour coming from the west. As of Thursday evening, testing was still being done on the samples, but the results of a second toxicity test are expected to be ready Friday. The APCD will release a final determination once all results are accounted for.The agency said that continued testing may not reveal anything if the smell is not present."We've definitely documented the odor, but we haven't to date been able to determine the source," said Jon Adams, chief of compliance for the agency.The air-pollution agency's screenings should be able to at least narrow down the cause of the malodorous but invisible fumes, providing a "fingerprint" of the responsible substance or compound, Adams noted. Such findings might not directly reveal the source, but could point investigators in the right direction, he said.Adams described the pervasive pall as an unusual happening."On occasion things like this happen, but it's pretty rare to this level," he said. "To impact such a wide area, that's pretty rare."People began making emergency calls about 2 p.m. Wednesday to report a pervasive and pungent smell variously described as akin to kerosene, diesel fuel, bus exhaust, lighter fluid and other petroleum-based substances, according to Maurice Luque, a spokesman for the San Diego Fire-Rescue Department.10News received dozens of calls from residents who said they smelled what they believed was jet fuel in the air.Residents from areas such as Encinitas, Solana Beach, Pacific Beach, Mira Mesa and La Jolla all reported the odor. Residents living in inland areas such as North Park, Hillcrest and Rancho Bernardo told 10News they smelled the odor in their area, as well. "I feel this air bothering my throat while walking," said Claudia from Encinitas.Andria in North Park added, "It is horrible here ... We are getting headaches and feeling really dizzy."There were no reports that the vapors led to any physical ailments, Luque said.A public affairs spokesman at MCAS Miramar said the odor could be smelled at the base, but noted that the base's air operations have had not reported any fuel spills or aircraft that had to dump jet fuel in the county area.An FAA spokesman told 10News they were not aware of any fuel dumping being carried out on Wednesday.Mario Aguilera of the Scripps Institution of Oceanography said he did not know of any algae blooms or anything organic that could be causing the smell.San Diego Gas & Electric officials said they have received numerous calls regarding the odor. The utility sent technicians into the community to try and determine the source of the smell.Engine crews investigated the widespread affected areas, including Del Mar Heights, Scripps Ranch, La Jolla, Hillcrest and downtown San Diego, finding no apparent source."They can't isolate what it is," Luque said in the early evening.10News checked with area hospitals to see if there was an increase of visits to their emergency rooms related to the smell, but all of those we spoke with said no.The unusual odor has prompted a lot of theories about its origin. One 10News viewer suggested it might have come from a leaking railroad tanker, seeping fumes as it crossed the county. Another, more complex theory is explained in a YouTube video that suggests the odor was caused by seismic activity, possibly signaling a major earthquake. "My gut feeling is that there is absolutely no truth to such a doomsday prediction," said San Diego State University seismologist, Kim Olsen. Olsen told 10News that while seismic activity can cause the earth to emit odors, the science behind this theory is "sensationalistic." After watching the video, Olsen said the theory that a series of seismic events from the Pacific Northwest to San Diego are linked is far-fetched. The unexplained phenomenon was similar to an episode that occurred about a year ago, when people in various San Diego beach areas began complaining of a lingering stink comparable to rotten eggs.Authorities eventually determined that decaying vegetation in and around estuaries and ocean inlets was to blame for the foul smell, Luque said.