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NEW YORK — Residents of the tri-state area were advised Wednesday to prepare for the first heat wave of the summer as high temperatures and humidity will make for sticky and at times risky conditions.

A heat warning has been issued by the National Weather Service for New York City, with heat-index values — or an estimation of how the temperature and humidity will feel combined — reaching the 90s for the five boroughs. The advisory is expected to end around 8 p.m. Thursday.

Highs could reach above 90 degrees through Friday. If we get three consecutive days of 90-plus readings, that would classify as an official heat wave.

Humidity is also expected to remain high through the work week, and showers and thunderstorms are possible Thursday.

In Philadelphia, an excessive heat warning has been issued, predicting a heat index value of around 100 during the afternoon through Friday.

High temperatures can lead to heat-related health issues, especially for the elderly and those in poor health, the weather service warned.

In order to lower risk, people are advised to reduce outdoor work and activity and schedule frequent rest breaks in shaded or air conditioned environments.

Children and pets should never be left unattended in a vehicle under any circumstances, as the interior can reach “lethal temperatures in a matter of minutes,” the weather service stated.

Pets should also be left in a cool environment and provided plenty of water.

Also on Wednesday, an air quality advisory was issued for New York City, Long Island and areas of western New York and the lower Hudson.

Ozone levels — or ground-level pollutants that are created when summer heat combines with pollution — are expected to increase.