During March of 2015, two critics episodes of PM2.5 were observed in Medellín-Colombia (levels over 75 µg/m3: three times the recommended by the World Health Organization, for three days or more). A case- crossover study stratified over time was carried out to establish the effects of these episodes in the use of emergency services for respiratory and cardiovascular events. Conditional logistic regression models were used to estimate the relative risks with 95% confidence intervals. Data of atmospheric conditions, particulate matter and the diagnostics in emergencia services were collected from public environmental information system and the individual records of health service provision. The analysis was made in populations under 5 years old and and adults aged 65 years and over. In under 5 years old population every 10 μg/m³ increase in PM2.5 was associated with increases in the risk percentage of emergency room visits for asthma (2.8%), acute respiratory infection (2.0%) and pneumonia (2.2%) in the same day of exposure, and for acute respiratory infection (3.6%) in the first day of exposure. For adults 65 and older the increase of 10 μg/m³ in PM2.5 were associated to excess risks for emergencies room visits due to COPD complications in the first day of exposure (0.8%), cerebrovascular disease after two days of (7.2%) and acute respiratory infection in the same day of exposure and after seven days of cumulative exposure with increases in risk pecentages of 0.6% and 0.5% respectively.These results confirm clear associations between ambient PM2.5 and negative health effects in respiratory and cardiovascular systems during critc episodes in Colombia