Is the air generally dirty in your neighborhood? Do you live near a mill or polluting factory? Is this only occuring this winter?
Check that the oil boiler heat exchanger, flue to chimney, and chimney itself are not causing the problem?
Other sources (fireplace, kerosene heaters, etc.)?
If its only happening this winter, and you can rule out all other factors, I would have the boiler looked at again.
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