Abstract. Compared to conventional cage production systems, cage-free (CF) hen housing offers hens more space and opportunities to exercise their natural behaviors (e.g., perching, dust bathing, and foraging). However, CF housing poses a number of inherent environmental challenges, among which are high levels of particulate matter (PM) and ammonia (NH3). Spraying water on CF hen house litter (e.g., 125 mL m-2 per cm of litter depth) has been shown to mitigate generation of PM by 60% to 70% in our previous lab-scale tests. The objectives of this study were to verify the lab-scale findings of PM reduction in a commercial CF hen house in central Iowa and to evaluate the indoor air quality (e.g., PM and NH3 concentrations) and litter moisture content as affected by water spray. The commercial CF house had a nominal capacity of 50,000 laying hens and measured 154 m × 21.3 m × 3.0 m (L × W × H). A water sprinkling system was installed in half of the hen house in the length direction (treatment section), while the other half of the hen house served as the control. For each of the three trials conducted during the winter of 2017-2018, spray dosage (125 mL H2O m-2 per cm of litter depth) was set according to the initial litter depth before spraying. Results show that PM concentration was reduced by 37% to 51% in the treatment section of the CF hen house. The lower reduction efficiency in the field experiment than in the lab-scale tests was partially attributed to the fact that water spray in the commercial hen house was applied only to the open litter area, and not the litter area under the aviary system, due to limited space. Adjusting the spray dosage according to litter depth is necessary for maintaining an appreciable reduction efficiency. Litter moisture content in the treatment section was 9% to 14% higher than in the control section (15.6% vs. 14% in trial 1, 14.6% vs. 12.2% in trial 2, and 17.7% vs. 14.9% in trial 3), but NH3 concentrations in the treatment and control sections were similar during the field experiment. Keywords: Air quality, Alternative hen housing, Dust, Litter moisture content.