Table 1.

Summary of the epidemiological studies on the adverse health effects of ambient UFPs.

Targeting system	Method	Effect	Indicator	Main results	Reference
Respiratory system	Randomized crossover study	Asthma in adulthood	FEV1	Exposure to UFPs was considerably associated with FEV1 (HR = −1.52, 95% CI: −2.28, −0.77).	[13]
	Cohort study	Asthma during childhood	Asthma incidence	During the entire pregnancy period, the risk of childhood asthma exacerbation increased by a OR of 4.28 (95% CI: 1.41, 15.7) for every doubling of exposure concentration to UFPs.	[15]
	Semiexperimental design	Acute airway inflammation	FeNO	For every increase of one IQR in UFPs (33,000 particles/cm3), there is a corresponding increase of 11% (95% CI: 5%, 17%) in FeNO levels immediately after exposure and 12% (95% CI: 6%, 17%) in FeNO levels 2 h after exposure relative to the baseline.	[12]
	Cohort study	COPD	COPD incidence	Each IQR increase in ambient UFPs was associated with incident COPD (HR = 1.06, 95% CI: 1.05, 1.09). However, when adjusted for NO2, the association weakened, and the HR no longer increased (HR = 1.01, 95% CI: 0.98, 1.03).	[16]
	Cohort study	Respiratory disease	Respiratory mortality	There was a delayed and prolonged association between UFPs and increased respiratory system mortality (9.9%, 95% CI: −6.3%, 28.8%), which was associated with an average increase of 2,750 particles/cm3 over 6 days.	[17]
Cardiovascular system	Cohort study	Cardiovascular disease	Cardiovascular mortality	The association between UFPs and ischemic heart disease mortality was the strongest (daily increase in mortality rate of 7.1%, 95% CI: 2.9%, 11.5%, for every increase of 6,250 particles/cm3).	[24]
	Cross-sectional study	Microvascular dysfunction	MVF	Personal exposure to UFPs during outdoor activities was significantly negatively correlated with MVF (decreased by 1.3% per IQR, 95% CI: 0.1%, 2.5%) and pulse amplitude, and positively correlated with leukocyte and neutrophil counts.	[27]
	Cross-sectional study	Microvascular dysfunction	MVF	There was a significant negative correlation between MVF and outdoor UFPs (MVF decreases by 9% for every increase in IQR value).	[28]
	Cross-sectional study	Cardiovascular disease	CRP	An increase in the IQR (2,000 particles/cm3) of UFP was associated with a 6.3% increase (95% CI: 0.4%, 12.5%) in high sensitivity CRP levels.	[25]
Digestive system	Cohort study	liver cancer	Liver cancer incidence	Positive linear association was observed between black carbon [HR = 1.15, 95% CI: (1.00, 1.33) per 0.5 × 10−5/m] and liver cancer incidence.	[41]
Central nervous system	Cohort study	Malignant brain cancer	Malignant brain cancer mortality	The overall risk of malignant brain cancer increased by 12% (95% CI: −2%, 27%) among all participants with each IQR increase in airport-related UFP exposure (6,700 particles/cm3).	[56]
	Cohort study	Incident brain tumors	Brain tumors incidence	After adjusting for confounding factors, there was a positive correlation between UFPs and incidence of brain tumors, with a HR of 1.133 (95% CI: 1.032, 1.245) for every increase of 10,000/cm3	[57]
	Case-crossover design	Ischemic stroke	Hospitalization for ischemic stroke	Exposure to UFPs lead to a 21% increase (95% CI: 4%, 41%) in the number of hospitalizations for mild ischemic stroke without AF.	[58]

AF, atrial fibrillation; CI, confidence interval; COPD, chronic obstructive pulmonary disease; FEV₁, forced expiratory volume in 1 sec; IQR, interquartile range; MVF, microvascular function; OR, odds ratio.