Here's the english summary of a Swedish report, funded by the Swedish Energy Agency. The complete report is available in Swedish here:

http://www.afabinfo.com/pdf_doc/fou_rapporter/AntioxidantrapportCTH.pdf

Summary
Increased residential wood burning has been questioned referring to environmental and health
hazards due to emitted smoke components. In this project, the presence of phenolic
antioxidants in wood smoke was demonstrated, presenting a more positive aspect on the
smoke.

The antioxidants are mainly methoxyphenols released from the lignin of the wood.
Dimethoyxyphenols from hardwood are the strongest antioxidants. At combustion
temperatures below 800oC and especially for smouldering wood the methoxyphenols normally
constitute the main fraction of the organic smoke components. Most methoxyphenols condense
on cooling and are present as particulate matter in ambient air. The phenolic antioxidants are
released together with almost as large amounts of 1,6-anhydroglucose formed mainly from
cellulose.

The assessment of components in wood smoke which are positive or at least harmless with
respect to health has met with great interest. The occurrence and proportions of specific
methoxyphenols in wood smoke of various origins have therefore been emphasized in the
project. Comparisons were also made with smoke for food curing, from newspaper burning and
from burning of forest biomass components.

Smoke from residential fireplace burning of hardwood in particular consists of a large
proportion of effective antioxidants and a low proportion of hazardous compounds. Residential
boilers with unsatisfactory combustion may produce a smoke with elevated concentrations of
benzene and polycyclic aromatic hydrocarbons. An environmentally labelled boiler emitted
almost negligible amounts of organic compounds. Wood pellets burnt in free-standing stoves
or in boiler burners emitted lower amounts of both antioxidants and hazardous compounds than
comparable firewood burning.