One in four real estate professionals surveyed by the analytics firm Clear Capital said the BP oil spill has caused home sales along the Gulf Coast to come to a "halt." More than half of the respondents who said the oil spill was having a negative impact also reported the incident had pushed home values down 5% to 15%. But the impact isn't just on values. More than 3% of respondents reported actual property damage from the spill. Many said sales have come to a stand still. In Mobile, Ala., home sales dropped 25% in June from last year, and other areas in southeastern Alabama and Florida reported decreased sales and property values because of the spill. In Panama City, Fla., professionals reported a 32.5% drop in sales volume from a year ago. "The results of our survey reflect the overall uncertainty of where and by how much the oil spill is affecting individual local markets," said Alex Villacorta, senior statistician at Clear Capital. "While social stigma appears to be the largest factor influencing the slowdown in home buying activity, it is clear the effects of the spill are being felt well inland from the coast." But the expiration of the homebuyer tax credit can't be spared blame, either. Pending home sales nationwide plummeted 30% in May after the expiration, according to the National Association of Realtors (NAR), and July home sales have dropped another 27% from last year, according to the real estate brokerage chain RE/MAX. "Many of these local markets in the Gulf have already experienced significant price declines over the last few years as well as a recent drop off in sales volume after the tax credit expiration," Villacorta said. "Additional downward pressure in the form of stigma and loss of employment will only serve to further dampen home price recovery." Other firms are trying to determine the effect the spill is having on real estate values. This week, Greenfield Advisors, which specializes in complex real estate valuation and advisory services, launched a new website to determine that impact. "Given that the data reveals a huge impact in housing markets due to real or perceived influences, the changing nature of public sentiment regarding the spill will undoubtedly drive market changes," according to the Clear Capital report. Write to Jon Prior.