Fate of 1 to 6 nm Candle Combustion Nanoparticles in Indoor Atmospheric Environments

Nano-organic carbon (NOC: 1-6 nm) particles are emitted during combustion processes and remain understudied down to their smallest sizes in indoor environments. We investigate the persistence of atmospheric nanoparticles in the NOC size range indoors during scented candle combustion in a residential test house. Our results indicate that atmospheric nanoparticles in the 1-6 nm size range dominate indoor particle concentrations, constituting up to ∼65% of total particles. The fate of these nanoparticles in indoor air is mainly governed by their apparent emission rates and the coagulation scavenging potential of the indoor atmosphere, parametrized through the net coagulation sink and aerosol Fuchs surface area. To better characterize this, we introduce a dimensionless parameter (S)─the ratio of the max-normalized net coagulation sink to the apparent emission rate─which effectively describes particle survival at specific sizes and offers a critical and previously lacking metric to evaluate nanoparticle persistence versus suppression in indoor environments. When this ratio exceeds 10, the concentration of 1-6 nm nanoparticles is suppressed by more than a hundred-fold. However, since this ratio typically remains <10 for most 1-6 nm nanoparticles, they persist at high concentrations, posing significant health risks due to their potential to penetrate deeply into the human respiratory tract, quantified through another novel parameter─the respiratory survival probability.