||Epidemiological research has emphasized the adverse effect of particulate matter on human health. Ultrafine particles (UFPs) have more severe effects than larger particles due to their smaller size. This research focuses on the study of human exposure to ultrafine particles in certain indoor environments. For estimation and analysis, a smoking room and a home, the two different indoor environments that are always under high ultrafine particle levels were considered. To predict the UFP concentrations and the induced human risk, the emission rates from major sources were investigated and a multizone model was developed and validated. Two sizes of smoking room equipped with displacement ventilation under variable fresh air supply rates were constructed to carry out the studies. The ultrafine particle exposure in the smoking rooms was measured by an ultrafine particle counter, and very high levels were found. A manikin installed on a motorized rail was used to study the effect of human movement on the leakage of the UFPs from the smoking room. A double-door anteroom design was incorporated into the smoking room to study its effectiveness in UFP leakage prevention. However, a significant leakage of UFP occurred when the smoker, simulated by the manikin installed on the rail, walked out of the room. The double-door anteroom design improved UFP leakage prevention but the problem still could not be completely mitigated. Exposure to airborne particulate matter (PM) emitted during cooking can lead to adverse health effects. An understanding of the exposure to PM during cooking at home provides a foundation for the quantification of possible health risks. The concentrations of airborne particles covering the ultrafine (14.6–100 nm) and accumulation mode (100–661.2 nm) size ranges and PM2.5 during and after cooking activity were measured in 20 homes in Hong Kong. 12 of them with naturally ventilation and no smoking activity were chosen for more detailed analysesanalyze. The monitored homes all practiced Chinese-style cooking. Cooking elevated the average number concentrations of ultrafine particles (UFPs) and accumulation mode particles (AMPs) by 10 fold from the background level in the living room and by 20–40 fold in the kitchen. Cooking emitted particles dispersed quickly from the kitchen to the living room indicating that the health impact is not limited to people in the kitchen. Particles in cooking emissions were mainly in the ultrafine size range in terms of the number count while AMPs contributed to at least 60% of the surface area concentrations in the kitchen and 73% in the living room. This suggests that AMPs could still be a major health concern since the particle surface area concentration is suggested to have a more direct relationship with inhalation toxicity than with number concentration. Combustion activities such as smouldering cigarettes, incense burning and cooking are important sources of particulate matter in indoor environments. Vacuum cleaning is a very common activity in homes, and contributes to the non-combustion-related sources of particulate matter. UFPs can be generated from the motor of a vacuum cleaner, and can be resuspended from the floor during vacuum cleaning. In this study we investigated the rates at which ultrafine particles are emitted from cigarettes, incense and vacuum cleaners in a small test chamber. UFP emissions from cooking were obtained by conducting experiments in a residential kitchen. Some experiments were carried out in a household living room to find emission rates of UFPs in terms of number concentration and surface area. Three different scenarios of vacuum cleaning were investigated, including Normal Condition, Filter Removed, and Filter and Dust Bag Removed. Particle number concentrations and size distributions from these sources were measured using a scanning mobility particle sizer and the UFP emission rates were then determined using a material balance approach. Using a filter and dust bag in the vacuum cleaner can both remove the UFPs. A filter has higher removal efficiency than a dust bag for the UFPs. A database of emission rates for UFP sources can be compiled, which will be useful in estimating UFP concentrations and subsequent human exposure. A multizone model was used to predict the UFP concentrations in a household setting over time. Some data from the experiments of cigarette smouldering and incense burning were used to verify this model. Residual and Kolmogorov-Smirnov (KS) tests were employed to quantify the goodness-of-fit. The results showed a good fit between the measured data and the simulation results. This multizone model is a good approach to predict variations of UFP concentrations in indoor environments. The dose response of people in different zones can be estimated easily based on this model. Particulate matter generated during cooking contained various carcinogens. These particles consist of both ultrafine particles (nanoparticles) and coarse particles. Exposure and risk assessment studies often use particle mass concentration as dosimetry, which ignores the impact of ultrafine particles due to their insignificant mass compared to coarse particles. This study aims at characterizing the cancer risk with regard to cooking-generated particulate matter using homes in Hong Kong as an example. A risk assessment scheme modified from an existing risk model was developed to consider the cancer risk contributed from both fine ultrafine and coarse particles. Exposure assessment was conducted based on particle concentration data measured in Hong Kong homes. 16 homes without smoking were chosen from the 20 site measurement set. The predicted cancer risk was then compared to the cancer incidence rate in Hong Kong to examine its appropriateness. It is found that the ultrafine particles contribute a much higher risk than that of coarse particles and the modified risk assessment scheme gives an estimate much closer to the incidence rate than the conventional scheme. Use of a grease extractor cannot completely contain the particles and a significant portion of particles can be transported from kitchens to other regions of the homes. The modified risk assessment scheme can serve as a tool in assessing environmental quality as well as setting up design and ventilation guidelines and exposure standards regarding particulate matter.